Deuteronomy 15:19

When the Lord thy God hath blessed thee in all thou doest, every firstling of thy herds, and thy flocks, even all the males, thou shalt dedicate to the Lord thy God. Thou shalt not work a firstling bull, nor shear the firstlings of thy flock.

Every first-born that shall be born among thy kine and thy sheep, thou shalt sanctify the males to the Lord thy God; thou shalt not work with thy first-born calf, and thou shalt not shear the first-born of thy sheep.