It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that when we used to participate in the battles with the Prophet (peace and blessings be upon him), some of us would fast and some would not. However, the one who fasted would not consider himself superior to the one who did not fast, nor would the one who did not fast consider himself superior to the one who fasted. (The meaning is that whoever had the strength to fast would do so, and whoever did not have the strength would leave it and make it up later.)