Musnad Ahmad 31:96

Isma'il narrated to us, al-Jurayri informed us, from Abu Nadra, from Abu Sa'id al-Khudri, who said: "We used to go on military expeditions with the Messenger of Allah, may Allah bless him and grant him peace, and among us were those who were fasting and those who were not fasting. The one who was fasting would not find fault with the one who was not fasting, and the one who was not fasting would not find fault with the one who was fasting. They considered that—meaning whoever found strength and fasted—that was good. And they considered that whoever found weakness and did not fast, that was good."