Deuteronomy 25:4-6

American Standard Version

⁴ Thou shalt not muzzle the ox when he treadeth out the grain.

⁵ If brethren dwell together, and one of them die, and have no son, the wife of the dead shall not be married without unto a stranger: her husband’s brother shall go in unto her, and take her to him to wife, and perform the duty of a husband’s brother unto her. ⁶ And it shall be, that the first-born that she beareth shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel.