Genesis 38:8-9

Then Judah told Onan, “Go in to your brother’s widow, and perform your duty as a brother-in-law [under the levirate marriage custom]; [be her husband and] raise children for [the name of] your brother.” [Deut 25:5-10] Onan knew that the child (heir) would not be his [but his dead brother’s]; so whenever he lay with his brother’s widow, he spilled his seed on the ground [to prevent conception], so that he would not give a child to his brother.