Deuteronomy 15:19
All the firstling males that come of thy herd and of thy flock thou shalt sanctify unto the LORD thy God: thou shalt do no work with the firstling of thy bullock, nor shear the firstling of thy sheep. All the firstling males that come of thy herd and of thy flock thou shalt sanctify unto the LORD thy God: thou shalt do no work with the firstling of thy bullock, nor shear the firstling of thy sheep.
The command stands among the laws rehearsed for Israel on the eve of crossing the Jordan to enter the promised land. It belongs to the sacred law given at that time, whose lessons were pressed upon the people to be remembered and kept.
The firstling males of herd and flock were to be set apart to the Lord, with no work to be done with the firstling of the bullock and no shearing of the firstling of the sheep. The animals thus assigned, whose flesh the priests were permitted to eat, were young. Joined with the requirement that sacrificial animals be without blemish, this confines the meat so permitted to lean meat taken from healthy, young animals.