Sahih Muslim Book 4, Hadith Number 757.

Chapter : He who hears the Adhin should respond like it, invoke blessings upon the Apostle (may peace be upon him)and then beg for him the Wasila.

A Hadith like it has been narrated by Abu Huraira but for these words: “He (the man saying the prayer) does not know how much he has prayed.