It is narrated from Hazrat Abu Saeed (may Allah be pleased with him) that we used to travel with the Prophet (peace and blessings be upon him) in the month of Ramadan, so some of us would fast and some would not fast, but the one who fasted would not find fault with the one who did not fast, nor would the one who did not fast find fault with the one who fasted. (This means that when a person had the strength to fast, he would do so, and if he did not have the strength, he would leave it and make it up later.)