Genesis 38:6-9

6And Judah took a wife for Er his firstborn, and her name was Tamar.

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

8Then 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.

9But 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.