Al-Baqarah 2:185

Shahru ramadana allatheeonzila feehi alqur-anu hudan linnasiwabayyinatin mina alhuda walfurqanifaman shahida minkumu ashshahra falyasumhu waman kanamareedan aw AAala safarin faAAiddatun min ayyaminokhara yureedu Allahu bikumu alyusra wala yureedubikumu alAAusra walitukmiloo alAAiddata walitukabbiroo AllahaAAala ma hadakum walaAAallakum tashkuroon

The month of Ramaḍān [is that] in which was revealed the Qur’ān, a guidance for the people and clear proofs of guidance and criterion. So whoever sights [the crescent of] the month, let him fast it; and whoever is ill or on a journey - then an equal number of other days. Allāh intends for you ease and does not intend for you hardship and [wants] for you to complete the period and to glorify Allāh for that [to] which He has guided you; and perhaps you will be grateful.

The month of Ramadan is the one in which the Qur’ān was revealed as guidance for mankind, and as clear signs that show the right way and distinguish between right and wrong. So those of you who witness the month must fast in it. But the one who is sick, or is on a journey (should fast) as much from other days (as he missed). Allah intends (to provide) ease for you and does not intend (to create) hardship for you. All this is so that you may complete the number (of fasts as prescribed) and proclaim the Takbīr of Allah for having guided you, and (so) that you may be grateful.