Observing Saum (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, 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.
(2:184)
Those on a Journey
What is a Journey? When is One Considered a Traveller? - AbuKhadeejah.com
Very comprehensive!
The Ruling on a Sick or Traveling Person Fasting : Imaam ibn Al-‘Uthaimeen - the second portion discusses traveling
Fasting Rulings Related to Travel (audio - 37:28) - Dr. Saleh us Saleh
Can he shorten his prayers and eat before leaving his house on a journey?
Can he shorten his prayers and eat before leaving his house on a journey?
Those Who are Ill
The Ruling on a Sick or Traveling Person Fasting: Imaam ibn Al-‘Uthaimeen - the first portion discusses traveling
Fasting Rulings Related to Sickness (audio - 25:34) - Dr. Saleh us Saleh
(He gets into the discussion of various types of medicines towards the end and whether they break the fast)