Genesis 38:1-10

About that time, Judah left his brothers and went to stay with a man named Hirah in the town of Adullam. There Judah met a Canaanite girl, the daughter of a man named Shua, and married her. Judah had sexual relations with her, and she became pregnant and gave birth to a son, whom Judah named Er. Later she gave birth to another son and named him Onan. Still later she had another son and named him Shelah. She was at Kezib when this third son was born. Judah chose a girl named Tamar to be the wife of his first son Er. But Er, Judah’s oldest son, did what the LORD said was evil, so the LORD killed him. Then Judah said to Er’s brother Onan, “Go and have sexual relations with your dead brother’s wife. It is your duty to provide children for your brother in this way.” 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.