The Prophet ﷺ said, "Every good deed performed by the son of Adam is multiplied from ten times to seven hundred times. Allah says, 'Except for fasting, for it is for Me and I shall reward it. He gives up his food for My sake, his drink for My sake, and his desire for My sake. So it is for Me, and I shall reward it.' Fasting is a shield. The fasting person has two moments of joy, one when he breaks his fast, and another when he meets his Lord. The smell that comes from the mouth of the fasting person, changed from the effect of food, is more fragrant to Allah than the scent of musk."
حدثنا محمد بن جعفر حدثنا شعبة عن سليمان عن ذكوان
عن النبي ﷺ قال كل حسنة يعملها ابن آدم عشر حسنات إلى سبع مائة حسنة يقول الله عز وجل إلا الصوم هو لي وأنا أجزي به يدع الطعام من أجلي والشراب من أجلي وشهوته من أجلي فهو لي وأنا أجزي به والصوم جنة وللصائم فرحتان فرحة حين يفطر وفرحة حين يلقى ربه ولخلوف فم الصائم حين يخلف من الطعام أطيب عند الله من ريح المسك
Its isnad is sahih. It was mentioned previously at 9675. His saying "when it changes" means when the smell of his mouth appears, which is the odor from the mouth after Dhuhr.