Musannaf Ibn Abi Shayba 21:356

Zayd ibn al-Hubab narrated to us, Malik ibn Anas informed me, he said: Sumayy, the freed slave of Abu Bakr, informed me, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever says in a day one hundred times: '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,' it will be for him equivalent to freeing ten slaves, and one hundred good deeds will be written for him, and one hundred evil deeds will be erased from him, and it will be a protection for him from Satan for the rest of his day until nightfall. And no one will come with anything better than what he has brought, except for one who says more than that."