Sahih Muslim
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Book Number: 035 / Hadith Number: 6586
Abu Dharr reported that Allah's Messenger (may peace be upon him) was asked as to which words were the best. He said: Those for which Allah made a choice for His Angels and His servants (and the words are):" Hallowed be Allah and praise is due to Him."