Deuteronomy 25:4-5

4Do not tie shut the mouth of your ox while it walks on your grain to prepare it for you.

5Perhaps two brothers live together in the same place. One of the brothers may die before he has any son. If that happens, his widow must not marry anyone who does not belong to the family. Instead, the other brother must marry her. That is the right thing for him to do for his dead brother.