Deuteronomy 25:5-6
5
If brothers dwell together, and one of them die, and have no son, the wife of the dead shall not be married outside to a stranger: her husband's brother shall go in to her, and take her to him as wife, and perform the duty of a husband's brother to her.
6
It shall be, that the firstborn whom she bears shall succeed in the name of his brother who is dead, that his name not be blotted out of Israel.