Sahih Bukhari Volume 008, Book 075, Hadith Number 412.

Narated By Abu Huraira : Allah's Apostle said," Whoever says: "La ilaha illal-lah wahdahu la sharika lahu, lahu-l-mulk wa lahu-l-hamd wa huwa 'ala kulli shai'in qadir," one hundred times will get the same reward as given for manumitting ten slaves; and one hundred good deeds will be written in his accounts, and one hundred sins will be deducted from his accounts, and it (his saying) will be a shield for him from Satan on that day till night, and nobody will be able to do a better deed except the one who does more than he."