Genesis 38:8-10

International Standard Version

⁸ So Judah instructed Onan, “You are to have sexual relations with your dead brother’s wife, performing the duty of a brother-in-law with her, and have offspring for your brother.”

⁹ But Onan knew that the offspring wouldn’t be his own heir, so whenever he had sexual relations with his brother’s wife, he would spill his semen on the ground to avoid fathering offspring for his brother. ¹⁰ The Lord considered what Onan was doing to be evil, so he put him to death, too.