Genesis 38:6-9

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

7But Er, Judah's firstborn, was wicked in the sight of the Lord, and the Lord put him to death.

8Then Judah said to Onan, "Go in to your brother's wife and perform the duty of a brother-in-law to her, and raise up offspring for your brother."

9But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the semen on the ground, so as not to give offspring to his brother.