Sahih Bukhari Volume 008, Book 075, Hadith Number 393.
Narated By Abu Musa : We were in the company of the Prophet on a journey, and whenever we ascended a high place, we used to say Takbir (in a loud voice). The Prophet said, "O people! Be kind to yourselves, for you are not calling upon a deaf or an absent one, but You are calling an All-Hearer, and an All-Seer." Then he came to me as I was reciting silently, "La haul a wala quwwata illa bil-lah." He said, "O 'Abdullah bin Qais! Say: La haul a walaquwata illa bil-lah, for it is one of the treasures of Paradise." Or he said, "Shall I tell you a word which is one of the treasures of Paradise? It is: La haul a wala quwwata illa bil-lah."