Deuteronomy 25:4-6

Darby Translation

⁴ Thou shalt not muzzle the ox when he treadeth out [the corn].

⁵ If brethren dwell together, and one of them die, and have no son, the wife of the dead shall not marry a stranger abroad: her husband's brother shall go in unto her, and take her to him as wife, and perform the duty of a husband's brother unto her.

⁶ And it shall be, that the firstborn that she beareth shall stand in the name of his brother who is dead, that his name be not blotted out from Israel.