The Messenger of Allah ﷺ said, "By the One in whose hand my soul is, the smell of the fasting person's mouth is more pleasing to Allah than the fragrance of musk. He leaves his desire, his food, and his drink for my sake. Fasting is for me, and I myself reward it. Every good deed is rewarded tenfold, up to seven hundred times over, except for fasting, which is for me, and I myself reward it."
أن رسول الله ﷺ قال: والذي نفسي بيده لخلوف فم الصائم أطيب عند الله من ريح المسك، إنما يذر شهوته وطعامه وشرابه من أجلي فالصيام لي وأنا أجزي به كل حسنة بعشر أمثالها إلى سبعمائة ضعف إلا الصيام فهو لي وأنا أجزي به.
Malik narrated it in Siyam (58), from Abu Zinad, from Araj, from Abu Hurairah, and he related it.Bukhari narrated it in Siyam (1894) through Malik, with the same account.Muslim narrated it in Siyam (1151:162) through Mughira Hizami, from Abu Zinad, with his chain of transmission.The wording given is Bukhari's, while Muslim's wording is very abridged.