Sahih Muslim 1116 d
The Book of Fasting
It is permissible to fast or not to fast during Ramadan for one who is travelling for no sinful purpose, if his journey is two stages or further. But it is better for the one who is able to fast without suffering any harm to do so, and the one for whom it is difficult may break the fast
Abu Sa'id al-Khudri (Allah be pleased with him) reported: We went out on an expedition with the Messenger of Allah (ﷺ) during Ramadan. Some of us observed the fast and some of us broke it. Neither the observer of the fast had any grudge against one who broke it, nor the breaker of the fast had any grudge against one who had fasted They knew that he who had strength enough (to bear its rigour) fasted and that was good, and they also found that he who felt weakness (and could not bear the burden) broke it, and that was also good.