Genesis 38:6-9

6Now Judah took a wife for Er his firstborn; her name was Tamar.

7But Er, Judah’s firstborn, was evil in the sight of the Lord, and the Lord killed him [in judgment].

8Then 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.”

9Onan 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.