Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: We used to go on military campaigns with the Messenger of Allah (peace and blessings be upon him) in Ramadan, and some of us were fasting and some were not, but no fasting person found fault with one not fasting, nor one not fasting with a fasting person. They held that whoever had strength and fasted had done well, and whoever felt weakness and broke the fast had also done well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8168