The Messenger of Allah ﷺ said, "Your Lord says, 'Every good deed is rewarded tenfold, up to seven hundred times. But fasting is for Me, and I Myself reward it. Fasting is a shield from the Fire. The breath of the one who fasts is more fragrant to Allah than the scent of musk. If someone behaves foolishly toward one of you while he is fasting, let him say, I am fasting.'"
حدثنا عمران بن موسى القزاز حدثنا عبد الوارث بن سعيد حدثنا علي بن زيد عن سعيد بن المسيب
قال رسول الله ﷺ إن ربكم يقول كل حسنة بعشر أمثالها إلى سبع مائة ضعف والصوم لي وأنا أجزي به الصوم جنة من النار ولخلوف فم الصائم أطيب عند الله من ريح المسك وإن جهل على أحدكم جاهل وهو صائم فليقل إني صائم
[Sahih]. Ahmad narrated it (2/414) from the hadith of Abdul-Warith bin Saeed with the same wording, and its basis is agreed upon, Bukhari (no. 5927) and Muslim (no. 1151/163, 164). The hadith has corroborating narrations.On this subject there are also narrations from Muaz bin Jabal [to come, no. 2616], Sahl bin Saad [to come, no. 765], Kaab bin Ujra [Bukhari in Tarikh al-Kabir 7/220, and Hakim 4/153], Salama bin Qaysar [Tabarani in Kabir 7/56, no. 6365], and Bashir bin Khasasiyyah [Tabarani in Kabir 2/45, no. 1235].