But Onan knew that the heir would not be his; and it came to pass, when he went in to his brother's wife, that he emitted on the ground, lest he should give an heir to his brother.
Genesis 38:9
Surrounding context
38:8 And Judah said to Onan, "Go in to your brother's wife and marry her, and raise up an heir to your brother."
38:9 But Onan knew that the heir would not be his; and it came to pass, when he went in to his brother's wife, that he emitted on the ground, lest he should give an heir to his brother.
38:10 And the thing which he did displeased the LORD; therefore He killed him also.