Deuteronomy 25:5-7

5“If two brothers are living together on the same property and one of them dies without a son, his widow may not be married to anyone from outside the family. Instead, her husband’s brother should marry her and have intercourse with her to fulfill the duties of a brother-in-law.

6The first son she bears to him will be considered the son of the dead brother, so that his name will not be forgotten in Israel.

7“But if the man refuses to marry his brother’s widow, she must go to the town gate and say to the elders assembled there, ‘My husband’s brother refuses to preserve his brother’s name in Israel—he refuses to fulfill the duties of a brother-in-law by marrying me.’