Musnad Ahmad 937:239

Yazid narrated to us, al-Jariri Abu Mas'ud informed us, from Abu Abdullah al-Jasri, from Ibn al-Samit, from Abu Dharr, who said: I said: "O Messenger of Allah, which speech is most beloved to Allah, the Almighty and Majestic?" He said: "That which He chose for His angels: 'Glory be to Allah and by His praise, glory be to Allah and by His praise' (Subhan Allah wa bihamdihi, Subhan Allah wa bihamdihi), you say it three times."