Genesis 38:6-9

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

7And Er, Judah’s firstborn, was wicked in the sight of Yhwh; And Yhwh slew him.

8And Judah said unto Onan, Go in unto thy brother’s wife, and marry her, and raise up seed to thy brother.

9And Onan knew that the seed should 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 that he should give seed to his brother.