Musnad Ahmad 30:1539

Hassan narrated to us, Zuhayr narrated to us, Abu Balj narrated to us, that ‘Amr ibn Maymun narrated to him, saying: Abu Hurayrah said to me: The Messenger of Allah (may Allah bless him and grant him peace) said to me: "O Abu Hurayrah, shall I not guide you to a word from the treasure of Paradise?" He said: I said, "Yes, may my father and mother be ransomed for you." He said: "Say: 'There is no power except by Allah' (La hawla wa la quwwata illa billah)."