Genesis 38:6-9

And Judah took a wife for Er his first-born, and her name was Tamar. And Er, Judah’s first-born, was wicked in the sight of Jehovah; and Jehovah slew him. And Judah said unto Onan, Go in unto thy brother’s wife, and perform the duty of a husband’s brother unto her, and raise up seed to thy brother. And Onan knew that the seed would not be his; and it came to pass, when he went in unto his brother’s wife, that he spilled it on the ground, lest he should give seed to his brother.