Deuteronomy 25:5-6

⁵“If brothers dwell together, and one of them dies and has no son, the widow of the dead man shall not be married to a stranger outside the family; her husband’s brother shall go in to her, take her as his wife, and perform the duty of a husband’s brother to her. ⁶And it shall be that the firstborn son which she bears will succeed to the name of his dead brother, that his name may not be blotted out of Israel.