Genesis 38:7-9

Living Bible

⁷ But Er was a wicked man, and so the Lord killed him.

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.