Exodus 22:30

Seven days it shall be with his dam - For the mother's health it was necessary that the young one should suck so long; and prior to this time the process of nutrition in a young animal can scarcely be considered as completely formed. Among the Romans lambs were not considered as pure or clean before the eighth day; nor calves before the thirtieth: Pecoris faetus die octavo purus est, bovis trigesimo - Plin. Hist. Nat., lib. viii.