מדרש על בכורות 51:36

(Devarim 15:19) "Every firstling (bechor)": Scripture apprises us that a bechor is to be eaten (by the Cohein) all of its (first) year. This tells me only of an unblemished bechor. Whence do I derive (the same for) a blemished one (that cannot be sacrificed)? From "every bechor." From "You shall not work with the bechor of your bullock, etc." we are taught that it is forbidden to shear or to work with a bechor. This tells me only of an unblemished bechor. Whence do I derive (the same for) a blemished one? From "every bechor."