Genesis 38:8-10

Living Bible

⁸ Then Judah said to Er’s brother, Onan, “You must marry Tamar, as our law requires of a dead man’s brother; so that her sons from you will be your brother’s heirs.”

⁹ But Onan was not willing to have a child who would not be counted as his own, and so, although he married her,^[a] whenever he went in to sleep with her, he spilled the sperm on the bed to prevent her from having a baby which would be his brother’s. ¹⁰ So far as the Lord was concerned, it was very wrong of him to deny a child to his deceased brother, so he killed him, too.