The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Muslim :: Book 35 : Hadith 6500
A hadith like this has been transmitted on the authority of A'mash with the same chain of transmitters and he (further) said: There is for him ten like that (the good he performed) or more than that.