Leviticus 20:10 - 21
ESV - 10 If a man commits adultery with the wife of his neighbor, both the adulterer and the adulteress shall surely be put to death. 11 If a man lies with his father's wife, he has uncovered his father's nakedness; both of them shall surely be put to death; their blood is upon them.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.