Tractate Niddah - Chapter 10 - Mishnah 3 נידה פרק י׳ משנה ג׳

If a zav or a zavah examined themselves on the first day and found themselves clean and on the seventh day and found themselves clean, but did not examine themselves during the other intervening, days: Rabbi Eliezer says: they are in a presumptive condition of cleanness. Rabbi Joshua says: they are entitled [to count as clean] only the first day and the seventh day. Rabbi Akiva says: they are entitled to reckon as clean the seventh day alone.