Rashi on Leviticus 15:13/רש"י על ויקרא ט״ו:י״ג

1 וכי יטהר means WHEN HE CEASETH [FROM HIS ISSUE] (Sifra, Metzora Parashat Zavim, Chapter 5 1).

2שבעת ימים לטהרתו [THEN SHALL HE NUMBER TO HIMSELF] SEVEN DAYS FROM THE DATE WHEN HE CEASED — i. e. seven days clean (free) from the uncleanness resulting from the issue, i. e. in which he sees no issue, and all of them (of the seven days) must be consecutive (Sifra, Metzora Parashat Zavim, Chapter 5 6).