Bereshis 38:8-10

Orthodox Jewish Bible

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.

¹⁰ And the thing which he did was rah in the eyes of Hashem; therefore He slaughtered him also.