BT Pes 55a(21) :מעיקרא תאני מנהגא לבסוף תאני אסורא‏ : initially [i.e. at the beginning of the Mishna] he learns (that it is) a custom, and at the end he learns (that it is) a prohibition
See the entire entry for minhāḡ, minhāḡā n.m. custom