From Sahih Muslim (Hadith 2704):
Abu Huraira reported Allah's Messenger (ﷺ) as saying:Allah, the Majestic and the Exalted, said: Every act of the son of Adam is for him except fasting. It is done for My sake, and I will give a reward for it. By Allah in Whose Hand is the life of Muhammad, the breath of the observer of fast is sweeter to Allah than the fragrance of musk
Reference: Muslim 2704