Cal. Food & Agric. Code § 23801

A person shall not buy, sell, or accept a horse, mule, burro, or sheep, the carcass of any such animal from which the hide or skin has not been removed, or the hide or skin of such animal, unless the seller or donor gives, and the buyer or donee receives, at the time of delivery of the animal, carcass, or hide, a written bill of sale or written instrument, which states the number, kind, and brand or brand and marks of each such hide, skin, carcass, or animal, that is signed by the party that gives it.