Exodus 13:12

that thou shalt set apart unto the Lord all that openeth the matrix, and every firstling that cometh of a beast which thou hast; the males shall be the Lord's. The female children and the female young were not included in this order, but the male first-born, both of man and beast, was to be offered and consecrated to the Lord. Cf Exodus 22:29; Deuteronomy 15:21.