Courtesy:http://prramamurthy1931.blogspot.in/...-ramayana.html


परदाराभिमर्शात्तु नान्यत् पापतरं महत्
Paradaaraabhimarshaattu naanyat paapataram mahat


3.38.30


There is no greater sin than coveting another man's wife.


[Context: This is Maaricha's warning to Ravana who is bent on abducting Sita and solicits the help of Maaricha in this misadventure.]