Genesis 38:8-9

Then Judah said to Er’s brother Onan, “Go and have sexual relations with your dead brother’s wife. It is your duty to provide children for your brother in this way.” But Onan knew that the children would not belong to him, so when he was supposed to have sexual relations with Tamar he did not complete the sex act. This made it impossible for Tamar to become pregnant and for Er to have descendants.