The Messenger of Allah ﷺ said, "Whoever says, '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,' ten times, it will be equivalent for him to freeing ten slaves," or he said, "a slave."
حدثنا يزيد أخبرنا داود عن عامر عن عبد الرحمن بن أبي ليلى
أن رسول الله ﷺ قال من قال لا إله إلا الله وحده لا شريك له له الملك وله الحمد وهو على كل شيء قدير عشر مرات كن له كعدل عتق عشر رقاب أو رقبة
Its isnad is sahih, its narrators are trustworthy and well known. Yazid is Ibn Harun, Dawud is Ibn Abu Hind, and Amir is Shaybi. The hadith has already appeared at 18440.