On the authority of ʿAlī ibn Abī Ṭālib, from the Messenger of Allah (peace and blessings be upon him), who said: "Allah, the Blessed and Exalted, says: Fasting is for Me, and I shall reward for it. And the fasting person has two moments of joy: one when he breaks his fast, and one when he meets his Lord. By Him in Whose Hand is my soul, the smell of the mouth of the fasting person is more pleasing to Allah than the fragrance of musk."(1)

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(1) Sound due to corroborating evidence. This chain contains an error made by al-ʿAlāʾ — namely Ibn Hilāl — for Shuʿbah narrated it, as in the following narration, from Abū Isḥāq, from Abū al-Aḥwaṣ, from ʿAbd Allāh ibn Masʿūd, as a statement of his (mawqūf). The author said, as quoted from him by al-Mizzī in al-Tuḥfah (10166): "This is the correct version in our view, and the hadith of al-ʿAlāʾ is an error. I have seen narrations from al-ʿAlāʾ that are rejected (manākīr)." ʿUbayd Allāh is Ibn ʿAmr al-Raqqī, Zayd is Ibn Abī Unaysah, and Abū Isḥāq is ʿAmr ibn ʿAbd Allāh al-Sabīʿī. It appears in al-Sunan al-Kubrā under number (2532).
It is corroborated by the hadith of Abū Saʿīd that follows under number (2213).

Al-Sindī said: Regarding his statement, "Fasting is for Me, and I shall reward for it," several meanings have been mentioned, but what accords with the hadiths is that it is an allusion to the greatness of its reward and that it has no limit. This is what the contrast in the hadith indicates: "No good deed does the son of Adam perform except that it is written for him as ten good deeds up to seven hundred times over, except fasting, for it is for Me, and I shall reward for it" [and it will follow under number (2215)]. This also accords with the saying of Allah the Exalted: {Indeed, the patient will be given their reward without account} [al-Zumar: 10]. This is because its being singled out from among all other deeds as being exclusively for One of immense greatness, whose greatness has no end and no limit, and that this Immense One is the One who undertakes its reward — from this the mind naturally concludes that its reward is something without limit. It may also be said, on this basis, that the meaning of His saying "for Me" is: I alone know the measure of its reward and its multiplication. And by this, the contrast becomes apparent between it and His saying: "Every deed of the son of Adam is for him, except fasting — it is for Me." That is, every deed of his, in the sense that he knows its reward and the measure of its multiplication in general terms, as Allah has explained regarding it, except fasting, for it is the patience whose reward has not been assigned any limit; rather, He said: {Indeed, the patient will be given their reward without account}. It is also possible to say that the meaning of his statement, "Every deed of the son of Adam is for him..." etc., is that all the deeds of the son of Adam fall under the category of servitude and service, and are thus fitting for him and proportionate to his state — unlike fasting, for it falls under the category of abstaining from eating and drinking and being free of need for that, and thus falls under the category of assuming the character traits of the Lord, the Blessed and Exalted. As for the hadith, "No good deed does the son of Adam perform..." etc., on this meaning it requires an implicit assumption, such that one says: Every deed of the son of Adam has a limited reward because it is for him, that is, commensurate with his measure — except fasting, for it is for Me, so its reward is not confined, rather I am the One who undertakes its reward according to My measure. And Allah knows best.