The 77th mitzvah is that we are commanded that every leper must bring an offering after his condition heals.
It consists of three animals — a burnt-offering, a sin-offering, and a guilt-offering — and one log of oil. If he is poor, his offering consists of a sheep as the guilt-offering and two doves or two young pigeons — one for a burnt-offering and the other for a sin-offering. This is the fourth category of "those whose atonement is incomplete" [as long as the sacrifice has not yet been brought].
The source of this commandment is G‑d's statement (exalted be He), "On the eighth day, he shall take two unblemished male sheep and one unblemished yearling female sheep."
Perhaps someone will ask: why don't you count the sacrifices brought by all "those whose atonement is incomplete" as one mitzvah? This is because they all share the same idea — that they lack atonement, and would then be [one mitzvah] like one of the purification procedures. You would then say: "the following mitzvah is the commandment not to consider the purification of certain categories complete until the sacrifices have been brought — [those categories being] zav, zavah, a woman who has just given birth, and a leper. This seems to be similar to counting the purification of the mikvah as one mitzvah, that each person who is tameh must fulfill, regardless of the type of impurity. So too, you could count the sacrifices of "those who lack atonement" as one mitzvah, regardless of the type of impurity!
G‑d knows that this would be absolutely true if the identical sacrifice was brought for each of the four categories of "those whose atonement is incomplete." Then it would be similar to purification with water, which is a purification procedure which applies to [virtually] all who are tameh. But since, as you can see, the type of sacrifice varies, we are forced to count each one separately; because what completes the purification for this category does not work for another category. These categories are like the water of the red heifer, a mikvah, and the four species used to purify a leper — which, although they are all used for purification, nevertheless count as three mitzvos, as will be later explained.
The laws regarding the four categories of "those whose atonement is incomplete" and the laws of their sacrifices are discussed — in general and in detail — in the first two chapters of tractate Kerisus, the second chapter of Erachin and Zevachim, the 8th chapter of Nazir, the end of Negaim, tractate Kinim, and sprinkled throughout various passages in the Talmud. The vast majority, though, are found in the places I've noted.
