Genesis 38:7-9

Complete Jewish Bible

⁷ But ‘Er, Y’hudah’s firstborn, was evil from Adonai’s perspective, so Adonai killed him. ⁸ Y’hudah said to Onan, “Go and sleep with your brother’s wife — perform the duty of a husband’s brother to her, and preserve your brother’s line of descent.” ⁹ However, Onan knew that the child would not count as his; so whenever he had intercourse with his brother’s wife, he spilled the semen on the ground, so as not to give his brother offspring.