The Messenger of Allah ﷺ said, "Every good deed performed by the son of Adam is rewarded ten to seven hundred times over. Allah said, 'Except for fasting, for it is Mine and I shall reward it. He leaves his desire and his food for My sake.' Fasting is a shield. The one who fasts has two joys, a joy when he breaks his fast and a joy when he meets his Lord. The breath of the one fasting is more fragrant to Allah than the scent of musk."
أخبرنا إسحق بن إبراهيم قال أنبأنا جرير عن الأعمش عن أبي صالح
عن رسول الله ﷺ قال ما من حسنة عملها ابن آدم إلا كتب له عشر حسنات إلى سبع مائة ضعف قال الله عز وجل إلا الصيام فإنه لي وأنا أجزي به يدع شهوته وطعامه من أجلي الصيام جنة للصائم فرحتان فرحة عند فطره وفرحة عند لقاء ربه ولخلوف فم الصائم أطيب عند الله من ريح المسك
Narrated by Muslim, Book of Fasting, Chapter on the Excellence of Fasting, no. 164/1151, from the hadith of Jareer bin Abdul-Hameed, and by Bukhari, Book of Tawheed, Chapter on the saying of Allah Almighty, "They wish to change the words of Allah", no. 7492, from the hadith of Amash, through the same chain. It is also found in al-Kubra, no. 2525.