Musnad Ahmad 788:120

Muhammad ibn Ja'far narrated to us, 'Uthman ibn Ghiyath narrated to us, from Abu 'Uthman, from Abu Musa, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do you know?" or "Shall I guide you to a treasure from the treasures of Paradise?" He said: "Allah and His Messenger know best." He said: "There is no power and no strength except by Allah (La hawla wa la quwwata illa billah)."