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 dominion and to Him belongs all praise, and He is capable of all things,' it will be equal in reward to freeing ten slaves, one hundred good deeds will be recorded for him, one hundred sins will be erased from him, and it will be a protection for him from Satan that day until evening. No one will come with anything better than what he has brought, except someone who did more than that."
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، وكتبت 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.
Malik narrated it in Kitab al-Quran (486) from Sumayy, the freedman of Abu Bakr, from Abu Salih Samman, from Abu Hurairah, and he related it.Bukhari narrated it in Bad al-Khalq (3293), and Muslim in al-Dhikr wal-Dua (2691), both of them through Malik, with the same wording.For more, see Kitab al-Adiyah wal-Adhkar.