Deuteronomy 25:4-6

New Century Version

⁴ When an ox is working in the grain, do not cover its mouth to keep it from eating.

⁵ If two brothers are living together, and one of them dies without having a son, his widow must not marry someone outside her husband’s family. Her husband’s brother must marry her, which is his duty to her as a brother-in-law. ⁶ The first son she has counts as the son of the dead brother so that his name will not be forgotten in Israel.