{o you who believe! observing as-sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become al-muttaqoon (the 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 can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a miskeen (needy person) (for every day). but whoever does good of his own accord, it is better for him. and that you fast 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 sawm (fasts) that month, and whoever is ill or on a journey, the same number [of days which one did not observe sawm (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 takbeer (allahu akbar: allah is the most great)] for having guided you so that you may be grateful to him.},

[a-baqarah: 183-185]

from the benefits of the ayahs

1) allah has obligated fasting upon the believers as he had obligated it upon those before them (from the jews and the christians) for that which exists therein of benefits in this world and the hereafter; 

2) the days of fasting are limited and do not exceed thirty (30) days; 

3) it is permissible for the one who is ill and (also for) the traveler to abstain from fasting and thereby make up those days missed as a result, after ramadan;

4) it used to be that one could choose between fasting in ramadan or paying fidyah (instead of fasting), and this was then abrogated, and fasting became obligatory;

5) the ayahs mention the excellence of the month of ramadan and the excellence of the qur'an which allah revealed in ramadan. since the revelation occurred from up above to mankind below, then this is indicative of allah's loftiness above his throne, as the ayahs and the authentic ahaadeeth have mentioned; 

6) fasting is obligatory upon whoever sights the moon for the beginning of the month of ramadan, except for one who is ill or the traveler.