Genesis 38:6-9

In the course of time, Judah arranged for his firstborn son, Er, to marry a young woman named Tamar. But Er was a wicked man in the LORD’s sight, so the LORD took his life. Then Judah said to Er’s brother Onan, “Go and marry Tamar, as our law requires of the brother of a man who has died. You must produce an heir for your brother.” But Onan was not willing to have a child who would not be his own heir. So whenever he had intercourse with his brother’s wife, he spilled the semen on the ground. This prevented her from having a child who would belong to his brother.