Musnad Ahmad 26:584

'Ali ibn Hafs narrated to us, Waraqa' narrated to us, from Mansur, from Abu Wa'il, from 'Amr ibn Sharhabil, from 'Abdullah, who said: I said: "O Messenger of Allah, which sin is the greatest?" He mentioned it, then he recited: "And those who do not invoke with Allah another deity" [Al-Furqan: 68] — up to — "disgraced" [Al-Furqan: 69].