Sahih Bukhari
Sahih Bukhari Volume 005, Book 057, Hadith Number 099.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 57. Companions of the Prophets
Narated By Qais : Bilal said to Abu Bakr, "If you have bought me for yourself then keep me (for yourself), but if you have bought me for Allah's Sake, then leave me for Allah's Work."