Genesis 38:6-9

Then Judah took a wife for Er his firstborn, and her name was Tamar. But Er, Judah’s firstborn, was wicked in the sight of the LORD, and the LORD killed him. And Judah said to Onan, “Go in to your brother’s wife and marry her, and raise up an heir to your brother.” But Onan knew that the heir would not be his; and it came to pass, when he went in to his brother’s wife, that he emitted on the ground, lest he should give an heir to his brother.