Genesis 38:7-9

Contemporary English Version

⁷But Er was very evil, and the Lord took his life. ⁸So Judah told Onan, “It's your duty to marry Tamar and have a child for your brother.”^[a]

⁹Onan knew the child would not be his,^[b] and when he had sex with Tamar, he made sure that she would not get pregnant.

38.8 It's your duty … child … brother: If a man died without having children, his brother was to marry the dead man's wife and have a child, who was to be considered the child of the dead brother (see Deuteronomy 25.5,6).
38.9 the child … not be his: When Judah died, Onan would get his dead brother's share of the inheritance, but if his dead brother had a son, the inheritance would go to him instead.