Musnad Ahmad 788:91

'Affan narrated to us, Hammad narrated to us, from Thabit al-Bunani, and 'Ali ibn Zayd, and al-Jariri, from Abu 'Uthman al-Nahdi, from Abu Musa al-Ash'ari, that the Messenger of Allah (may Allah bless him and grant him peace) said to him: "Shall I not guide you to a treasure from the treasures of Paradise?" He said: "And what is it?" He said: "There is no power and no strength except by Allah (La hawla wa la quwwata illa billah)."