Genesis 38:7

5She yet again bore a son, and named him Shelah. He was at Chezib when she bore him.

6Judah took a wife for Er, his firstborn, and her name was Tamar.

7Er, Judah’s firstborn, was wicked in the LORD’s sight. So the LORD killed him.

8Judah said to Onan, “Go in to your brother’s wife, and perform the duty of a husband’s brother to her, and raise up offspring for your brother.”

9Onan knew that the offspring wouldn’t be his; and when he went in to his brother’s wife, he spilled his semen on the ground, lest he should give offspring to his brother.