Musnad Ahmad 30:3552

Rawh narrated to us, Malik narrated to us, from Sumayy, the freed slave of Abu Bakr ibn Abd al-Rahman, from Abu Salih al-Samman, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever says 'Subhan Allah wa bihamdihi' (Glory be to Allah and His praise) one hundred times in a day, his sins will be removed, even if they were like the foam of the sea."