Musnad Ahmad 30:1306

Bakr ibn Isa narrated to us, Abu Awanah narrated to us, from Abu Balj, from Amr ibn Maymun, who said: Abu Hurayrah said: The Prophet of Allah, may Allah bless him and grant him peace, said to me: "O Abu Hurayrah, shall I not guide you to a word that is a treasure from the treasures of Paradise, under the Throne?" I said: "Yes, may my father and mother be ransomed for you." He said: "Say: 'La quwwata illa billah' (There is no power except with Allah)." Abu Balj said: And I think he said: "For Allah, the Almighty and Majestic, says: 'My servant has submitted and surrendered.'" He (the narrator) said: I said to Amr: Did Abu Balj say: Amr said: I said to Abu Hurayrah: "La hawla wa la quwwata illa billah" (There is no might and no power except with Allah)? He said: "No, it is in Surah al-Kahf: {And why, when you entered your garden, did you not say, 'What Allah willed; there is no power except with Allah?'} [Al-Kahf: 39]."