Musnad Ahmad 30:890

I read to Abd al-Rahman: Malik, 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: 'There is no god but Allah, alone, with no partner, to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things,' one hundred times in a day, it will be for him the equivalent of freeing ten slaves, and one hundred good deeds will be recorded for him, and one hundred evil deeds will be erased from him, and it will be a protection for him from Satan for that day until evening. And no one will bring anything better than what he has brought, except for a person who does more than that."