Genesis 38:9-11

But Onan knew that the children would not belong to him, so when he was supposed to have sexual relations with Tamar he did not complete the sex act. This made it impossible for Tamar to become pregnant and for Er to have descendants. The LORD was displeased by this wicked thing Onan had done, so the LORD killed Onan also. Then Judah said to his daughter-in-law Tamar, “Go back to live in your father’s house, and don’t marry until my young son Shelah grows up.” Judah was afraid that Shelah also would die like his brothers. So Tamar returned to her father’s home.