Genesis 38:7-9

International Standard Version

⁷ But the Lord considered Er, Judah’s oldest son, to be wicked—so he put him to death. 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.