كتاب الذكر والدعاء والتوبة والاستغفار
48  
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
(6)
Chapter: The Virtue Of Remembrance, Supplication, Drawing Close To Allah And Thinking Positively Of Him
(6)
باب فَضْلِ الذِّكْرِ وَالدُّعَاءِ وَالتَّقَرُّبِ إِلَى اللَّهِ تَعَالَى
Sahih Muslim 2687 b

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.
حَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، بِهَذَا الإِسْنَادِ ‏.‏ نَحْوَهُ غَيْرَ أَنَّهُ قَالَ ‏ "‏ فَلَهُ عَشْرُ أَمْثَالِهَا أَوْ أَزِيدُ ‏"‏ ‏.‏
Reference : Sahih Muslim 2687b
In-book reference : Book 48, Hadith 30
USC-MSA web (English) reference : Book 35, Hadith 6500
  (deprecated numbering scheme)