From Gensis chapter 38 8-10
8 Then Judah said to Onan, "Make love to your brother's wife. After all, you are her brother-in-law. So carry out your duty to her. Produce children for your brother."
9 But Onan knew that the children wouldn't belong to him. So every time he made love to his brother's wife, he spilled his semen on the ground. He did it so he wouldn't produce children for his brother.
10 What he did was evil in the Lord's eyes. So the Lord put him to death also.
8 Then Judah said to Onan, "Make love to your brother's wife. After all, you are her brother-in-law. So carry out your duty to her. Produce children for your brother."
9 But Onan knew that the children wouldn't belong to him. So every time he made love to his brother's wife, he spilled his semen on the ground. He did it so he wouldn't produce children for his brother.
10 What he did was evil in the Lord's eyes. So the Lord put him to death also.