Musnad Ahmad 30:891

I read to Abd al-Rahman: Malik, from Sumayy, the freed slave of Abu Bakr, 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 by His praise), one hundred times in a day, his sins will be erased, even if they were like the foam of the sea."