Sayyiduna Abu al-Darda' (may Allah be pleased with him) said: On some of the journeys with the Messenger of Allah (peace and blessings be upon him) we saw days that were extremely hot, so much so that one of us would place his hand on his head because of the intensity of the heat, and none of us was fasting except the Messenger of Allah (peace and blessings be upon him) and Sayyiduna Abdullah bin Rawahah (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8167
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
Sinan bin Salamah narrated from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever is on a journey on a mount, and comes to a place where he is well fed, let him fast wherever Ramadan reaches him."
Al-Bukhari said: Abd al-Samad bin Habib is munkar al-hadith (his narrations are rejected) and discarded; and al-Bukhari did not consider this hadith to be anything.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8169
Sayyiduna Anas (may Allah be pleased with him) said: "If I break the fast, it is Allah's concession; and if I fast, it is better."
This was also narrated from Hasan bin Salih with his chain as marfu' (from the Prophet), and it is nothing.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8170
Nafi' narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he said: "Breaking the fast of Ramadan on a journey is dearer to me than fasting."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8172