Genesis 38:7-9

Darby Translation

⁷ And Er, Judah's firstborn, was wicked in the sight of Jehovah, and Jehovah slew him.

⁸ Then Judah said to Onan, Go in to thy brother's wife, and fulfil to her the brother-in-law's duty, and raise up seed to thy brother.

⁹ But when Onan knew that the seed should not be his own, it came to pass when he went in to his brother's wife, that he spilled [it] on the ground, in order to give no seed to his brother.