Genesis 38:9-10 New Living Translation (NLT)
9 But Onan was not willing to have a child who would not be his own heir. So whenever he had intercourse with his brother’s wife, he spilled the semen on the ground. This prevented her from having a child who would belong to his brother. 10 But the Lord considered it evil for Onan to deny a child to his dead brother. So the Lord took Onan’s life, too.