Musnad Abu Ya'la al-Mawsili 152:99

On the authority of Abu Wa'il, from 'Amr ibn Sharhabil, from 'Abdullah, who said: A man said, "O Messenger of Allah, which sin is greatest in the sight of Allah?" He said, "That you call upon a rival for 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 it: "And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden, except by right..." [to the end of the verse].