Genesis 38:8

Revised Standard Version (RSV-CI)

Then Judah said to Onan, “Go in to your brother's wife, and perform the duty of a brother-in-law to her, and raise up offspring for your brother.”

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And the first-born said to the younger, “Our father is old, and there is not a man on earth to come in to us after the manner of all the earth.

You shall not uncover the nakedness of your brother's wife; she is your brother's nakedness.

But Naomi said, “Turn back, my daughters, why will you go with me? Have I yet sons in my womb that they may become your husbands?