Albert Barnes Commentary Exodus 8:26

"And Moses said, It is not meet so to do; for we shall sacrifice the abomination of the Egyptians to Jehovah our God: lo, shall we sacrifice the abomination of the Egyptians before their eyes, and will they not stone us?" — Exodus 8:26 (ASV)

The abomination - that is, an animal which the Egyptians considered sacrilegious to slay. The ox, bull, or cow, is meant. The cow was never sacrificed in Egypt, being sacred to Isis, and from a very early age the ox was worshipped throughout Egypt, and especially at Heliopolis and Memphis under various designations, Apis, Mnevis, Amen-Ehe, as the symbol or manifestation of their greatest deities, Osiris, Atum, Ptah, and Isis.