Genesis 38:8; Genesis 38:9

8 Then 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."

9 But 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.