Qur'an 2:185

The month of Ramadhan is when the Qur'an was sent down as guidance for mankind and with explanations for guidance, and as a Standard. Let any of you who is at home during the month, fast in it; while anyone who is ill or on a journey should [set an equal] number of other days. God wants things to be easy for you and does not want any hardship for you, so complete the number and magnify God because He has guided you, so that you may act grateful.