The Messenger of Allah ﷺ said, "Whoever says ten times, 'There is no god but Allah, the One, the Unique, the Eternal Refuge, who has taken no consort and no child, and to whom none is equal,' forty thousand good deeds will be recorded for him."
حدثنا إسحاق بن عيسى يعني الطباع قال حدثني ليث بن سعد قال حدثني الخليل بن مرة عن الأزهر بن عبد الله
قال رسول الله ﷺ من قال لا إله إلا الله واحدا أحدا صمدا لم يتخذ صاحبة ولا ولدا ولم يكن له كفوا أحد عشر مرات كتبت له أربعون ألف حسنة
Its isnad is daif because of Khalil bin Murrah. Bukhari and others graded him daif, and Ibn Kathir said in 8/543 that Ahmad narrated it uniquely. Haythami mentioned the hadith of Tabarani, in which it says a million good deeds, and he graded it daif because of Abul-Warqa, 10/85.