Musnad Ahmad 937:140

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj said: I heard Shu'bah, from Abu Mas'ud, from Abu 'Abd Allah al-Jasri, from 'Abd Allah ibn al-Samit, from Abu Dharr, from the Prophet of Allah, peace and blessings of Allah be upon him, that he said: "Indeed, the most beloved speech to Allah is for the servant to say: 'Glory be to Allah and by His praise' (Subhan Allah wa bihamdihi)." Hajjaj said: He asked the Prophet, peace and blessings of Allah be upon him, about the most beloved deed to Allah. Or the Prophet, peace and blessings of Allah be upon him, said: "Indeed, the most beloved speech to Allah is: 'Glory be to Allah and by His praise' (Subhan Allah wa bihamdihi)."