Genesis 38:9-11

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. But what he did was wicked in YAHWEH’s sight, so he took Onan’s life also. Then Judah said to his daughter-in-law Tamar, “Live as a widow in your father’s house until my youngest son Shelah grows up.” So Tamar went home to her parents, but Judah worried that Shelah would end up dead like his brothers.