Mishnah for Megillah 1:2

There are seven ways in which a zav is examined as long as he had not become subject to zivah: With regard to food, drink, as [to what] he had carried, whether he had jumped, whether he had been ill, what he had seen, or what he had thought. [It doesn't matter] whether he had thoughts before seeing [a woman], or whether he had seen [a woman] before his thoughts. Rabbi Judah says: even if he had watched beasts, wild animals or birds having intercourse with each other, and even when he had seen a woman's dyed garments. Rabbi Akiva says: even if he had eaten any kind of food, be it good or bad, or had drunk any kind of liquid. They said to him: Then there will be no zavim in the world!’ He replied to them: you are not held responsible for the existence of zavim!’ Once he had become subject to zivah, no further examination takes place. [Zov] resulting from an accident, or that was at all doubtful, or an issue of semen, these are unclean, since there are grounds for the assumption [that it is zivah]. If he had at a first [issue] they examine him; On the second [issue] they examine him, but on the third [issue] they don't examine him. Rabbi Eliezer says: even on the third [issue] they examine him because of the sacrifice.