Thou shall not covet thy neighbor's wife
More succinctly, "Thou shall not covet," is the 10th commandment.
"You shall not covet your neighbor's house. You shall not covet your neighbor's wife, or his manservant or maidservant, his ox or donkey, or anything that belongs to your neighbor." - Exodus 20:17
Though there is some disagreement as how the word 'covet' has been used, it is generally accepted that the 10th commandment, unlike the nine that come before it, concentrates on thought rather than deed. Jesus warns against all covetousness, as something that destroys a person from the inside.
"You covet and cannot obtain, so you fight and quarrel. You do not have, because you do not ask. You ask and do not receive, because you ask wrongly, to spend it on your passions. You adulterous people! Do you not know that friendship with the world is enmity with God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God." - James 4:2 through 4