“O you who believe! Observing As-Saum (fasting) is prescribed for you as it was prescribed for those before you, that you may be pious. [Observing Sawm (fasts)] for a fixed number of days, but if any of you is ill or on a journey, the same number (should be made up) from other days. And as for those who fast with difficulty (e.g. an old man, etc.), they have (a choice either to fast ) or to feed a Miskin (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast, it is better for you if only you know. The month of Ramadan in which was revealed the Qur’an, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Saum (fasts) that month, and whoever is ill or on a journey, the same number [of days which one did not observe Saum (fasts) must be made up] from other days. Allah intends for you ease, and He does not want to make things difficult for you. (He wants that you) must complete the same number (of days), and that you must magnify Allah [i.e. to say Takbir (Allahu-Akbar; Allah is the Most Great) on seeing the crescent of the months of Ramadan and Shawwal] for having guided you, so that you may be grateful to Him.” (Al- Baqarah: 183-185)

Fasting has several magnified benefits and abundant rewards. In order to extol its position, Almighty has linked fasting to Himself. In a Qudsi hadith reported by Abu Hurayrah, the Prophet ﷺ said: “Indeed, every righteous deed by the son of Adam is rewarded from ten times to seven hundred times. However Allah Almighty says: ‘Fasting is an exception. This is because it is for Me and I shall reward the one who fasts accordingly. He left his yearnings, wishes and food for My sake. ……... And, indeed, the change in odor (Al Khaloof) from his mouth is more pleasing to Allah than the fragrance of Musk (perfume).” [ Agreed upon.]
source: islamkingdom