Genesis 38:6-9

6Judah found a wife for his oldest son, Er. Her name was Tamar.

7But the Lord saw that Er, Judah's firstborn son, was very bad. So the Lord caused him to die.

8Then Judah said to Onan, ‘You should marry your dead brother's wife and have sex with her. You are her dead husband's brother. You must have a son who will become your dead brother's descendant.’

9But Onan did not want Tamar to give birth to his child. He knew that the child would not belong to him. So when he had sex with his brother's wife Tamar, he made his semen go onto the ground. Onan did this so that Tamar would not give birth to a child for his dead brother.