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 has power over all things,' it will be equivalent to freeing ten slaves, one hundred good deeds will be recorded for him, one hundred evil deeds will be erased from him, and it will be a shield for him from Satan throughout that day until nightfall. No one will come with anything better than what he has done, except for someone who said it more often."
حدثنا أبو بكر حدثنا زيد بن الحباب عن مالك بن أنس أخبرني سمي مولى أبي بكر عن أبي صالح
قال رسول الله ﷺ من قال في يوم مائة مرة لا إله إلا الله وحده لا شريك له له الملك وله الحمد وهو على كل شيء قدير كان له عدل عشر رقاب وكتبت له مائة حسنة ومحي عنه مائة سيئة وكن له حرزا من الشيطان سائر يومه إلى الليل ولم يأت أحد بأفضل مما أتى به إلا من قال أكثر
Bukhari narrated it in Bidal-Khalq, chapter on the description of Iblis and his hosts, hadith no. 6403 and 3293, and Muslim narrated it in Zikr wa Duaa, chapter on the merit of tahleel, tasbeeh, and duaa, hadith no. 26/91,28, from the hadith of Malik through this route, and it is also found in Muwatta, 1/209.