Genesis 38:9

But Onan knew that the offspring would not be his. So whenever he went in to his brother’s wife he would waste the semen on the ground , so as not to give offspring to his brother .

But Onan knew that the child would not be his; so whenever he slept with his brother`s wife, he spilled his semen on the ground to keep from providing offspring for his brother.

And 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.