Musnad Ahmad 937:48

Sufyan narrated to us, he heard Muhammad ibn al-Sa’ib ibn Barakah, from ‘Amr ibn Maymun, from Abu Dharr, who said: I was walking behind the Messenger of Allah, peace and blessings of Allah be upon him, and he said: "Shall I not guide you to a treasure from the treasures of Paradise?" I said: "Yes." He said: "There is no power and no strength except by Allah (La hawla wa la quwwata illa billah)."