Musnad Ahmad 788:95

Yahya narrated to us, from Uthman ibn Ghiyath, Abu Uthman narrated to us, from Abu Musa al-Ash'ari, from the Prophet, peace and blessings of Allah be upon him, who said: "Shall I guide you to a treasure from the treasures of Paradise?" or "Do you know what is a treasure from the treasures of Paradise?" I 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)."