Musnad Abu Ya'la al-Mawsili 152:30

Abu al-Rabi' narrated to us, Abu Shihab narrated to us, from al-A'mash, from Abu Wa'il, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked: "Which sin is greatest in the sight of Allah?" He said: "That you set up a rival to Allah, while He created you." He said: "Then what?" He said: "Then that you commit adultery with the wife of your neighbor." He said: So Allah revealed the confirmation of that: "And those who do not invoke with Allah another deity and do not kill the soul which Allah has forbidden, except by right, and do not commit unlawful sexual intercourse."