Midrash for Bekhorot 67:32

(Devarim 14:3) "You shall not eat any abomination": R. Eliezer says: Whence is it derived that if one slit the ear of a bechor (a first-born beast, thus blemishing it) and ate of it (the beast, after slaughtering it), he transgresses a negative commandment? From "You shall not eat any abomination."