הלכה על זבחים 199:14

The commandment of the matter of the impurity of the priests for their relatives, and included in it is that each one in Israel should mourn for their six relatives [the identify of which is] well-known: That the priests should become impure for the dead bodies (the relatives) that are mentioned in the Torah, as it is stated (Leviticus 21:3), "for her, he shall become impure." And this is a positive commandment, as so did the explanation come. And so is it explained in Sifra, Emor, Section 1:12, "'For her, he shall become impure' - is a commandment. If he does not want to become impure, we make him impure by force." And were it not that we received this explanation from our Sages, I would have reasoned to say that it be optional - if he wants, he becomes impure; if he does not want, he does not become impure - since Scripture prevented him from becoming impure for the rest of his relatives. And I would have said that regarding these mentioned in the section of the Torah, they were permitted to become impure if they wanted. Hence the explanation about it came to us - that it is not optional, but rather a commandment. And the Sages, may their memory be blessed, mentioned a story that Yosef the priest came when his wife died on the eve of Pesach and he did not want to become impure, and the Sages pushed him and made him impure by force. And Rambam, may his memory be blessed, wrote (Sefer HaMitzvot LaRambam, Mitzvot Ase 37), "And this itself is the commandment of mourning - meaning to say that each person in Israel is obligated to mourn for his relatives, meaning the six relatives mentioned in Scripture." And the verse that the Rabbi brought [as a source] (Mishneh Torah, Laws of Mourning 1:1) for the commandment of mourning is that which is stated by Aharon, "if I had eaten the sin-offering today, would it have been good in the eyes of the Lord?" (Leviticus 10:19). And he said, "And for the strengthening of this commandment did they elucidate about the priest that he is warned about impurity, that he should become impure regardless like other Israelites, in order that the laws of mourning not become weakened. And it was already elucidated that mourning of the first day is by Torah writ - and that is the day of death and burial. And they said in the elucidation in Moed Katan 14b [that] it is not practiced on the holiday - the positive commandment of the many comes and pushes off the positive commandment of the individual. And behold, it is elucidated [from this] that the obligation of mourning is from Torah writ and that it is a positive commandment - but only on the first day. And the remaining six [days] are rabbinic. And even a priest observes mourning on the first day, as he becomes impure for his relatives. And understand this." To here [are his words].