Bereshis 38:7-9

Orthodox Jewish Bible

⁷ And Er, bechor Yehudah, was rah in the eyes of Hashem; and Hashem slaughtered him.

⁸ And Yehudah said unto Onan, Go in unto eshet achicha, and enter into levirate marriage with her, and raise up zera to achicha.

⁹ And Onan knew that the zera should not be his; and it came to pass, when he went in unto eshet achiv, that he spilled it on the ground, lest he should give zera to achiv.