Rashi on Leviticus 14:2/רש"י על ויקרא י״ד:ב׳

1'תהיה תורת וגו ‎זאת THIS SHALL BE THE LAW [OF THE LEPER IN THE DAY OF HIS BEING PRONOUNCED CLEAN] — The words “in the day” inform us that we may not pronounce him clean at night (Sifra, Metzora, Section 1 3; Megillah 21a).