Sahih Bukhari Volume 004, Book 052, Hadith Number 235.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 52. Fighting for the Cause of Allah (Jihaad)
Narated By Abu Musa Al-Ashari : We were in the company of Allah's Apostle (during Hajj). Whenever we went up a high place we used to say: "None has the right to be worshipped but Allah, and Allah is Greater," and our voices used to rise, so the Prophet said, "O people! Be merciful to yourselves (i.e. don't raise your voice), for you are not calling a deaf or an absent one, but One Who is with you, no doubt He is All-Hearer, ever Near (to all things)."