Genesis 38:6-9

Judah got a wife for Er, his firstborn, and her name was Tamar. But Er, Judah’s firstborn, was wicked in the LORD’s sight; so the LORD put him to death. Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother.