Sahih Bukhari
Sahih Bukhari Volume 002, Book 022, Hadith Number 298.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 22. Actions While Praying
Narated By Mu'aiqib : The Prophet talked about a man leveling the earth on prostrating, and said, "If you have to do so, then do it once."