Sahih Bukhari Volume 008, Book 075, Hadith Number 418.

Narated By Abu Musa Al-Ash'ari : The Prophet started ascending a high place or hill. A man (amongst his companions) ascended it and shouted in a loud voice, "La ilaha illal-lahu wallahu Akbar." (At that time) Allah's Apostle was riding his mule. Allah's Apostle said, "You are not calling upon a deaf or an absent one." and added, "O Abu Musa (or, O 'Abdullah)! Shall I tell you a sentence from the treasure of Paradise?" I said, "Yes." He said, "La haul a wala quwwata illa billah."