Maliks Muwatta Book 3, Hadith Number 18.

Section : The Opening of the Prayer.

Yahya related to me from Malik from Ibn Shihab that AIi ibn Husayn ibn Ali ibn Abi Talib said, “The Messenger of Allah, may Allah bless him and grant him peace, used to say, ‘Allah is greater’ whenever he lowered himself and raised himself, and he continued to pray like that until he met Allah.”