Genesis 38:8-9

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.