Maliks Muwatta Book 15, Hadith Number 20.
Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr, from Abu Salih as-Samman from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “Whoever says ‘There is no god but Allah, alone, without any partner. The Kingdom and praise belong to Him and He has power over everything’ (La ilaha illa’llah, wahdahu la sharika lah, lahu’l mulku wa lahu’l hamd, wa huwa ala kulli shay’in qadir) one hundred times a day, it is the same for him as freeing ten slaves. One hundred good actions are written for him and one hundred wrong actions are erased from him, and it is a protection from Shaytan for that day until the night. No-one does anything more excellent than what he does except someone who does more than that.”