Reference for Bekhorot 50:55

אלא קדשים כיון דבדקו לן וקיימי דרכייהו למיפל בהו מומא

An objection was raised: If one plucks wool from an unblemished firstling, although there appeared on it subsequently a blemish and he slaughtered it, [the wool] is forbidden to be used.

Jastrow