Genesis 38:7-9

Young's Literal Translation

⁷ and Er, Judah's first-born, is evil in the eyes of Jehovah, and Jehovah doth put him to death.

⁸ And Judah saith to Onan, `Go in unto the wife of thy brother, and marry her, and raise up seed to thy brother;'

⁹ and Onan knoweth that the seed is not [reckoned] his; and it hath come to pass, if he hath gone in unto his brother's wife, that he hath destroyed [it] to the earth, so as not to give seed to his brother;