Sahih Bukhari Volume 001, Book 002, Hadith Number 040.
Sahih Bukhari Book 02. Belief
Narated By Abu Huraira : Allah's Apostle said, "If any one of
you improve (follows strictly) his Islamic religion then his good
deeds will be rewarded ten times to seven hundred times for
each good deed and a bad deed will be recorded as it is."