Deuteronomy 25:5-6

⁵"If brothers dwell together, and one of them dies and has no son, the wife of the dead shall not be married outside the family to a stranger; her husband's brother shall go in to her, and take her as his wife, and perform the duty of a husband's brother to her. ⁶And the first son whom she bears shall succeed to the name of his brother who is dead, that his name may not be blotted out of Israel.