The Messenger of Allah ﷺ said, "Whoever says one hundred times in a day, 'There is no god but Allah alone, with no partner. To Him belongs the kingdom and to Him belongs all praise, and He has power over all things,' it will be equivalent to freeing ten slaves, one hundred good deeds will be recorded for him, one hundred bad deeds will be erased from him, and it will be a protection for him from Satan that day until evening. No one will bring anything better than what he has brought, except someone who does more than that."
قال قرأت على عبد الرحمن: مالك عن سمي مولى أبي بكر بن عبد الرحمن عن أبي صالح السمان
أن رسول الله ﷺ قال من قال لا إله إلا الله وحده لا شريك له له الملك وله الحمد وهو على كل شيء قدير في يوم مائة مرة كانت له عدل عشر رقاب وكتب له مائة حسنة ومحيت عنه مائة سيئة وكانت له حرزا من الشيطان يومه ذلك حتى يمسي ولم يأت أحد بأفضل مما جاء به إلا أحد عمل أكثر من ذلك
Its isnad is sahih. It is in Muwatta, p. 209, and Bukhari related it in 6:243, 11:168-169 (Fath), and Muslim in 2:310, both through the route of Malik, from him. Tirmidhi and Ibn Majah also related it, as mentioned in Fath Kabir 3:221. See also what has come before in the Musnad of Abdullah bin Amr bin As, 6740, 7005.