Allah's Messenger (peace and blessing of Allah SWT may shower on him) 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 Allahi wa bi-hamdihi,
Subhan Allahil-'Azim."

Sahih Al-Bukhari Vol. 8 : No. 673 - Narrated by Abu Huraira (Radhiallaho anho)