Musannaf Ibn Abi Shayba 33:818

Yahya ibn Abi Bukayr narrated to us, from Shu'bah, from al-Jurayri, from Abu Abdullah al-Jasri, from Abdullah ibn al-Samit, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Shall I not inform you of the most beloved speech to Allah?" I said: "Yes, O Messenger of Allah (peace and blessings of Allah be upon him), inform me of the most beloved speech to Allah." He said: "The most beloved speech to Allah is: 'Subhan Allah wa bihamdihi' (Glory be to Allah and by His praise)."