Positive commandment · Mitzvat Aseh #74 of 613
For a zav to offer a sacrifice when he becomes purified [after his affliction]
להקריב הזב קרבן אחר שיטהר
The verse · Vayikra 15:13
וְכִי־יִטְהַר הַזָּב מִזּוֹבוֹ וְסָפַר לוֹ שִׁבְעַת יָמִים לְטׇהֳרָתוֹ וְכִבֶּס בְּגָדָיו וְרָחַץ בְּשָׂרוֹ בְּמַיִם חַיִּים וְטָהֵר
“And when he that has an issue is cleansed of his issue; then he shall number to himself seven days for his cleansing, and wash his clothes, and bathe his flesh in running water, and he shall be clean.”
The Rambam’s words
For a zav to offer a sacrifice when he becomes purified [after his affliction], as [Leviticus 15:13] states: "When a zav will become pure...."
להקריב הזב קרבן אחר שיטהר, שנאמר: "וכי יטהר הזב, מזובו" (ויקרא טו,יג).
Why · Sefer HaChinuch #179
Sacrifice of the zav when he is healed from his discharge
“From the roots of the commandment are what you will see at the end of my words. However, I am afraid of drawing close to the Dwelling of G-d, as I know that “anyone who comes close, who comes close” that has not sufficiently sanctified himself “will not see the House and live.””
Who & when
“And [it] is practiced at the time of the [Temple].”
Where in the Mishneh Torah
Hilchot Offerings for Those with Incomplete Atonement הלכות מחוסרי כפרה