Reference for Bekhorot 50:33

מחלוקת בשבדק ולא מצא וקמיפלגי בפלוגתא דר"מ ורבנן

The expression 'It shall be examined' means that an examination is made if the expert had permitted [the firstling] before the wool was torn away; in that case [the wool] is allowed, but if not, then it is not [allowed].

Jastrow