Abu Hurairah reported,

The Messenger of Allah ﷺ said, "By Him in whose hand my soul rests, the smell of the fasting person's mouth is more pleasing to Allah than the fragrance of musk. He says, 'He gives up his desire, his food, and his drink for My sake, so the fast is Mine and I Myself shall reward him for it. Every good deed is rewarded tenfold up to seven hundred times over, except fasting, for it is Mine and I Myself shall reward him for it.'"

وبإسناده أن رسول الله ﷺ قال والذي نفسي بيده لخلوف فم الصائم أطيب عند الله عز وجل من ريح المسك يقول إنما يذر شهوته وطعامه وشرابه من أجلي فالصوم لي وأنا أجزي به من كل حسنة بعشر أمثالها إلى سبع مائة ضعف إلا الصيام فهو لي وأنا أجزي به