Genesis 38:6-9

6Judah acquired a wife for Er his firstborn; her name was Tamar.

7But Er, Judah’s firstborn, was evil in the Lord’s sight, so the Lord killed him.

8Then Judah said to Onan, “Have sexual relations with your brother’s wife and fulfill the duty of a brother-in-law to her so that you may raise up a descendant for your brother.”

9But Onan knew that the child would not be considered his. So whenever he had sexual relations with his brother’s wife, he withdrew prematurely so as not to give his brother a descendant.