Genesis 38:5

There, Judah saw the daughter of a certain Canaanite man named Shua. He took her, and went in to her.

She conceived, and bore a son; and he named him Er.

She conceived again, and bore a son; and she named him Onan.

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

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

Judah 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.”