Sahih Muslim Book 31, Hadith Number 5914.
Chapter : The merits of ‘Ali b. Abi Talib (Allah be pleased with him).
Sa’d b. Abi Waqqas reported that Allah’s Messenger (may peace be upon him) left ‘Ali b. Abi Talib behind him (as he proceeded) to the expedition of Tabuk, whereupon he (‘Ali) said: Allah’s Messenger, are you leaving me behind amongst women and children? Thereupon he (the Holy Prophet) said: Aren’t you satisfied with being unto me what Aaron was unto Moses but with this exception that there would be no prophet after me.