Sahih Bukhari Volume 8, Book 78, Hadith Number 673.

Narated By Abu Huraira: Allah’s Apostle said, “(Following are) two words (sentences or utterances that are very easy for the tongue to say, and very heavy in the balance (of reward) and the most beloved to the Gracious Almighty (And they are): Subhan Allah wa bi-hamdihi; Subhan Allahi-l-‘Azim.”