GEN 38:9

9 And he knew that sons should not be born to him; and he entered [[in]] to the wife of his brother, and shedded his seed into the earth, lest the free children should be born by the name of the dead brother; (But Onan knew that any sons who would be born would not be his; so he slept with his brother’s wife, but he poured out his seed onto the ground, so that no children would be born in the name of his dead brother;)