Mishnayos Zavim Perek 1 Mishnah 6

If he saw one issue during the day and another at twilight, or one at twilight and the other the next day: If he knew that part of the issue occurred at day-time and part the next day, he is certain with regard to a sacrifice and uncleanness. But if it is in doubt whether part [of the issue] occurred at day-time and part on the next day he is certain with regard to uncleanness, but in one of doubt in with regard a sacrifice. If he had seen issues on two separate days at twilight, he is in doubt both with regard to defilement and with regard to a sacrifice. If [he had seen only] one issue at twilight, he is in doubt [also] with regard to [his] defilement.