Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 37
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 37, with English translation by Maimonides (Rambam).
להיות הכהנים מיטמאין לקרוביהם, ומתאבלין עליהן כשאר ישראל שהן מצווין להתאבל, שנאמר: "לָהּ יִטַּמָּא" (ויקרא כא, ג).
For the priests to become ritually impure and mourn for their relatives in the same manner as other Jews who are commanded to mourn, as [Leviticus 21:3] states: "He shall become impure for her."