Genesis 38:6-9

When their sons were grown, Judah arranged for Er, his oldest son, to marry a girl named Tamar. But Er had become so wicked in the sight of YAHWEH that YAHWEH ended his life. Then Judah said to Er’s brother Onan, “Your duty is to join yourself to her. Go perform your duty as a brother-in-law and provide an heir for your brother.” Onan, however, did not want to produce a child that would not be his own rightful heir, so whenever he and Tamar had intercourse, he purposely spilled his semen onto the ground to keep her from getting pregnant and having a child that would belong to his dead brother.