Deuteronomy 25:5-6

“When brothers live together and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a strange man. Her husband’s brother shall go in to her and take her to himself as wife and perform the duty of a husband’s brother to her. It shall be that the firstborn whom she bears shall assume the name of his dead brother, so that his name will not be blotted out from Israel.