Musnad Ahmad 31:481

'Ali ibn 'Asim narrated to us, Sa'id ibn Iyas informed us, from Abu Nadra, from Abu Sa'id, who said: "We used to travel with the Prophet, peace and blessings of Allah be upon him, during Ramadan. Among us were those who fasted, and among us were those who did not fast. The one who fasted would not criticize the one who did not fast, and the one who did not fast would not criticize the one who fasted."