Musannaf Ibn Abi Shayba 21:297

Zayd ibn al-Hubab narrated to us, Malik ibn Anas informed us, from Sumayy, from Abu Salih, from Abu Hurayrah, who said: 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."