Halacha

הלכה א
כֵּיצַד מְטַהֲרִים טְמֵא מֵת בְּמֵי נִדָּה. לוֹקֵחַ אָדָם טָהוֹר שְׁלֹשָׁה קְלָחִין שֶׁל אֵזוֹב וְאוֹגְדָן אֲגוּדָה אַחַת וּבְכָל בַּד וּבַד גִּבְעוֹל אֶחָד וְטוֹבֵל רָאשֵׁי גִּבְעוֹלִין בְּמֵי נִדָּה שֶׁבַּכְּלִי וּמִתְכַּוֵּן וּמַזֶּה עַל הָאָדָם אוֹ עַל הַכֵּלִים בַּיּוֹם הַשְּׁלִישִׁי וּבַיּוֹם הַשְּׁבִיעִי אַחַר שֶׁתָּנֵץ הַחַמָּה. וְאִם הִזָּה מִשֶּׁעָלָה עַמּוּד הַשַּׁחַר כָּשֵׁר. וְאַחַר שֶׁיַּזֶּה עָלָיו בַּיּוֹם הַשְּׁבִיעִי טוֹבֵל בַּיּוֹם וּמַעֲרִיב שִׁמְשׁוֹ וַהֲרֵי הוּא טָהוֹר לָעֶרֶב. טָבַל אֶת הָאֵזוֹב בַּלַּיְלָה וְהִזָּה בַּיּוֹם אוֹ שֶׁטָּבַל אֶת הָאֵזוֹב בַּיּוֹם וְהִזָּה בַּלַּיְלָה הַזָּאָתוֹ פְּסוּלָה וְהַמַּיִם מְטַמְּאִין מִשּׁוּם מֵי חַטָּאת כְּמוֹ שֶׁיִּתְבָּאֵר. עַד שֶׁתִּהְיֶה טְבִילַת הָאֵזוֹב בַּמַּיִם וְהַזָּאָתוֹ מִמֶּנּוּ בַּיּוֹם הַשְּׁלִישִׁי וּבַיּוֹם הַשְּׁבִיעִי אַחַר שֶׁתָּנֵץ הַחַמָּה וְאִם עָבַר וְעָשָׂה מִשֶּׁעָלָה עַמּוּד הַשַּׁחַר כָּשֵׁר כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
1.
How is a person who is impure because of contact with a human corpse purified by the water of sprinkling? A person who is pure takes three stalks of hyssop and binds them with one bond. Each stalk should have at least one bud. He should dip the top of the buds in the sprinkling water while it is in a container, focus his intent, and sprinkle it on the impure person or utensil on the third and seventh days after sunrise. If one sprinkled after dawn, it is acceptable. After the water has been sprinkled on a person on the seventh day, he should immerse in a mikveh during the day, wait until nightfall, and then, in the evening, he is pure.
If one dipped the hyssop [in the water] at night and sprinkled it during the day or dipped it [into the water] during the day and sprinkled it at night, the sprinkling is invalid. Moreover, the water imparts impurity just as the water of the ashes of the red heifer does, as will be explained.1As indicated by Chapter 15, Halachah 1, the water from an invalid sprinkling imparts impurity. [The sprinkling is not valid] unless the hyssop is dipped in the water and it is sprinkled from it on the third and seventh days after sunrise. If one transgressed and performed [these activities] after dawn, it is acceptable, as explained [above].

הלכה ב
מִי שֶׁנִּטְמָא בְּמֵת וְשָׁהָה כַּמָּה יָמִים בְּלֹא הַזָּאָה כְּשֶׁיָּבוֹא לְהַזּוֹת מוֹנֶה בְּפָנֵינוּ שְׁלֹשָׁה יָמִים וּמַזִּין עָלָיו בַּשְּׁלִישִׁי וּבַשְּׁבִיעִי וְטוֹבֵל בַּשְּׁבִיעִי וּמַעֲרִיב שִׁמְשׁוֹ. בַּמֶּה דְּבָרִים אֲמוּרִים בְּעַם הָאָרֶץ שֶׁבָּא לְהַזּוֹת שֶׁאֲפִלּוּ אָמַר הַיּוֹם שְׁלִישִׁי שֶׁלִּי אֵינוֹ נֶאֱמָן שֶׁמָּא הַיּוֹם נִטְמָא לְפִיכָךְ צָרִיךְ לִמְנוֹת בְּפָנֵינוּ. אֲבָל חָבֵר שֶׁבָּא לְהַזּוֹת מַזִּין עָלָיו אוֹ עַל כֵּלָיו מִיָּד. מִי שֶׁהֻזָּה עָלָיו בַּשְּׁלִישִׁי וְלֹא הֻזָּה עָלָיו בַּשְּׁבִיעִי וְשָׁהָה כַּמָּה יָמִים טוֹבֵל בְּכָל עֵת שֶׁיִּרְצֶה אַחַר הַשְּׁבִיעִי בֵּין בַּיּוֹם בֵּין בַּלַּיְלָה וּמַזִּין עָלָיו בַּיּוֹם בֵּין קֹדֶם טְבִילָה בֵּין אַחַר טְבִילָה אֲפִלּוּ טָבַל בְּלֵיל תְּשִׁיעִי אוֹ בְּלֵיל עֲשִׂירִי מַזִּין עָלָיו לְמָחָר אַחַר הָנֵץ הַחַמָּה:
כסף משנה
2.
When a person became impure due to a corpse and remained several days without having the water sprinkled upon him, when he comes to have the water sprinkled upon him, he should count three days in our presence. The water is sprinkled upon him on the third and seventh days. He immerses in a mikveh on the seventh day and waits until nightfall.
To whom does the above apply? To a common person who comes to have the water sprinkled upon him. Even if he says that this is the third day after he contracted impurity, his word is not accepted, for perhaps he became impure on this day. Therefore, he must count the days in our presence. If, by contrast, a chaver comes to have the water sprinkled upon him, it may be sprinkled on him and his implements immediately.
When such water was sprinkled on a person on the third day, but it was not sprinkled upon him on the seventh day, he may immerse in a mikveh whenever he desires after the seventh day, whether during the night or during the day and the water should be sprinkled on him during the day, whether before immersion or after immersion. Even if he immersed on the night of the ninth day or the night of the tenth day, the water may be sprinkled on him on the following day after sunrise.

הלכה ג
כָּל הַמְטַמְּאִין מְקַבְּלִין הַזָּאָה. כֵּיצַד. זָבִים וְזָבוֹת נִדּוֹת וְיוֹלְדוֹת שֶׁנִּטְמְאוּ בְּמֵת מַזִּין עֲלֵיהֶן בַּשְּׁלִישִׁי וּשְׁבִיעִי וַהֲרֵי הֵם טְהוֹרִין מִטֻּמְאַת מֵת אַף עַל פִּי שֶׁהֵן טְמֵאִין טֻמְאָה אַחֶרֶת. שֶׁנֶּאֱמַר (במדבר יט יט) "וְהִזָּה הַטָּהֹר עַל הַטָּמֵא בַּיּוֹם הַשְּׁלִישִׁי" הָא לָמַדְתָּ שֶׁהַזָּיָה מוֹעֶלֶת לוֹ אַף עַל פִּי שֶׁהוּא טָמֵא. וְכֵן הֶעָרֵל מְקַבֵּל הַזָּאָה. כֵּיצַד. עָרֵל שֶׁנִּטְמָא בְּמֵת וְהִזָּה עָלָיו שְׁלִישִׁי וּשְׁבִיעִי הֲרֵי זֶה טָהוֹר מִטֻּמְאַת מֵת. וּכְשֶׁיָּמוּל טוֹבֵל וְאוֹכֵל בְּקָדָשִׁים לָעֶרֶב:
כסף משנה
3.
It is acceptable for all of those who are impure to have this water sprinkled upon them. What is implied? The water may be sprinkled on zavim, zavot, women in the niddah state, women impure because of childbirth, on the third and seventh days after they contract the ritual impurity connected with a human corpse. After the sprinkling, they are purified from that impurity although they are still impure because of another type of impurity, as implied by Numbers 19:19: "And the pure person shall sprinkle on the impure," i.e., one may derive that the sprinkling is effective for him even though he is impure.
Similarly, a person who is uncircumcised may have this water sprinkled upon him. What is implied? If an uncircumcised person contracted the impurity connected with a corpse and had the water sprinkled upon him on the third and seventh days, he is pure from this impurity. When he is circumcised, he must immerse himself and may eat sacrificial foods in the evening.

הלכה ד
מִצְוַת אֵזוֹב שְׁלֹשָׁה קְלָחִין וְכָל קֶלַח וְקֶלַח גִּבְעוֹל אֶחָד. נִמְצְאוּ שְׁלֹשָׁה גִּבְעוֹלִין. וּשְׁיָרָיו שְׁנַיִם. אוֹ אִם לָקַח שְׁנַיִם בַּתְּחִלָּה וַאֲגָדָן כָּשֵׁר. נִתְפָּרְדוּ הַגִּבְעוֹלִין וְנָשְׁרוּ הֶעָלִין אֲפִלּוּ לֹא נִשְׁאַר מִכָּל גִּבְעוֹל מֵהֶם אֶלָּא כָּל שֶׁהוּא כָּשֵׁר שֶׁשְּׁיָרֵי הָאֵזוֹב בְּכָל שֶׁהוּא. קֶלַח שֶׁיֵּשׁ בּוֹ שְׁלֹשָׁה בַּדִּין מְפַסְּקוֹ וְאַחַר כָּךְ אוֹגֵד שְׁלָשְׁתָּן שֶׁמִּצְוָתוֹ לִהְיוֹת אֲגֻדָּה אַף עַל פִּי שֶׁלֹּא נִתְפָּרֵשׁ בַּתּוֹרָה שֶׁיִּהְיֶה אֲגֻדָּה. פְּסָקוֹ וְלֹא אֲגָדוֹ אוֹ שֶׁאֲגָדוֹ וְלֹא פְּסָקוֹ אוֹ שֶׁלֹּא פְּסָקוֹ וְלֹא אֲגָדוֹ וְהִזָּה בּוֹ כָּשֵׁר. הָאֵזוֹב הַקָּצָר אוֹגְדוֹ בְּחוּט עַל הַכּוּשׁ וְכַיּוֹצֵא בּוֹ וְטוֹבֵל בְּמַיִם וּמַעֲלֶה וְאוֹחֵז בָּאֵזוֹב וּמַזֶּה. נִסְתַּפֵּק לוֹ אִם מִן הַחוּט הִזָּה אוֹ מִן הַכּוּשׁ אוֹ מִן הַגִּבְעוֹל הַזָּאָתוֹ פְּסוּלָה:
כסף משנה
4.
The optimum way of fulfilling the mitzvah of the hyssop is to use three stalks and for each stalk to have one bud. If at first there were three buds and then only two remained or if one took two at first and bound them together, it is acceptable. Should the buds open and the leaves fall off, even if only the slightest portion of each remain, it is acceptable, because as long as the smallest amount remains from a hyssop, it is acceptable.
When a stalk has three buds, one should separate one from the other and afterwards, bind them together, for the mitzvah is for them to be bound, even though this is not explicitly stated in the Torah. If one separated them, but did not bind them, or bound them without separating them, or used them for sprinkling without separating them or binding them, the sprinkling is valid.
When a hyssop is short, one should bind it with a string on a weaving needle or the like, dip it into the water, lift it up, and then hold the hyssop in one's hand and sprinkle with it. If he did not do so, but instead, sprinkled with it while bound to the needle and he was uncertain whether he sprinkled from the string, from the needle, or from the bud, the sprinkling is invalid.

הלכה ה
אֵין מַזִּין לֹא בְּיוֹנְקוֹת הָאֵזוֹב וְלֹא בַּתְּמָרוֹת אֶלָּא בַּגִּבְעוֹלִין. אֵלּוּ הֵן הַיּוֹנְקוֹת גִּבְעוֹלִין שֶׁלֹּא גָּמְלוּ. וּמִי שֶׁהֻזָּה עָלָיו בְּיוֹנְקוֹת וְנִכְנַס לַמִּקְדָּשׁ פָּטוּר. וּמֵאֵימָתַי מַזִּין בָּאֵזוֹב מִשֶּׁיָּנֵץ. וְאֵזוֹב שֶׁהֻזָּה בּוֹ מֵי נִדָּה כָּשֵׁר לְטַהֵר בּוֹ אֶת הַמְצֹרָע. כָּל אֵזוֹב שֶׁיֵּשׁ לוֹ שֵׁם לְוַוי פָּסוּל. וְהָאֵזוֹב שֶׁקּוֹרִין אוֹתוֹ אֵזוֹב בְּיִחוּד הוּא הַכָּשֵׁר וְהוּא הָאֵזוֹב שֶׁאוֹכְלִין אוֹתוֹ בַּעֲלֵי בָּתִּים. אֲבָל זֶה שֶׁקּוֹרִין אוֹתוֹ אֵזוֹב יוֹן וְאֵזוֹב כַּחְלִית וְאֵזוֹב מִדְבָּרִי פָּסוּל:
כסף משנה
5.
One should not sprinkle with underdeveloped stalks of hyssop, nor with the seed pods, but with stalks. This is what is meant by underdeveloped stalks: stalks which have not reached maturity. Nevertheless, if the water of the ashes of the red heifer was sprinkled on a person with an underdeveloped hyssop and that person entered the Temple, he is not liable.
From which point should a hyssop be used for sprinkling? From the time it buds. When a hyssop has been used for the sprinkling of this water, it may be used to purify a person afflicted by tzara'at.
Whenever a hyssop is described by an additional term, it is not acceptable. The hyssop that is called a hyssop alone is the one which is acceptable. It is the hyssop that is eaten domestically. The species which are called Greek hyssop, red hyssop, desert hyssop, or Roman hyssop are invalid.

הלכה ו
אֵזוֹב שֶׁל אֲשֵׁרָה וְשֶׁל עִיר הַנִּדַּחַת וְשֶׁל עֲבוֹדָה זָרָה וְשֶׁל תְּרוּמָה טְמֵאָה פְּסוּלָה. שֶׁל תְּרוּמָה טְהוֹרָה לֹא יַזֶּה וְאִם הִזָּה כָּשֵׁר:
כסף משנה
6.
A hyssop that was worshiped as an asherah, came from a city that was led astray, belonged to a false deity, or came from impure terumah, is invalid. If it comes from pure terumah, as an initial preference, one should not sprinkle with it. If one did, the sprinkling is acceptable.

הלכה ז
אֵזוֹב שֶׁלִּקְּטוֹ לְעֵצִים וְנָפְלוּ עָלָיו מַשְׁקִין מְנַגְּבוֹ וְהוּא כָּשֵׁר לְהַזָּיָה. לִקְּטוֹ לָאֳכָלִין וְנָפְלוּ עָלָיו מַשְׁקִין אַף עַל פִּי שֶׁנִּגְּבוֹ פָּסוּל שֶׁהֲרֵי נִטְמָא לְהַזָּיָה שֶׁכָּל הַמַּשְׁקִין וְכָל הָאֳכָלִין וְכָל הַכֵּלִים הֲרֵי הֵן כִּטְמֵאִים לְעִנְיַן חַטָּאת כְּמוֹ שֶׁבֵּאַרְנוּ. לִקְּטוֹ לְחַטָּאת הֲרֵי זֶה כְּנִלְקָט לְעֵצִים וְאִם נָפְלוּ עָלָיו מַשְׁקִין מְנַגְּבוֹ וּמַזֶּה בּוֹ:
כסף משנה
7.
When a hyssop was gathered to be used for kindling and liquids fell upon it, one may dry it, and it will remain acceptable for sprinkling. If it was reaped for use as food, even if the water was dried, it is unacceptable, because it is considered as impure for sprinkling. For all liquids, foods, and implements are considered as impure with regard to the purification process using the water of the ashes of the red heifer, as will be explained. If a hyssop was reaped to use for the purification process, it is as if it was reaped for kindling. Thus if a liquid fell on it, it may be dried and used for sprinkling.

טהרה הלכות פרה אדומה פרק יא
Taharah Para Aduma Chapter 11