Musnad Ahmad 26:860

'Affan narrated to us, Mahdi narrated to us, Wasil al-Ahdab narrated to us, from Abu Wa'il, from Abdullah ibn Mas'ud, who said: I said: "O Messenger of Allah, which sin is the greatest?" He said: "That you set up an equal to Allah, while He created you." I said: "O Messenger of Allah, then what?" He said: "Then that you commit adultery with the wife of your neighbor."