Sayyiduna Abu Sa'id (may Allah be pleased with him) said: We travelled with the Messenger of Allah (peace and blessings be upon him) to Makkah while we were fasting. We stopped at a halting place, and the Messenger of Allah (peace and blessings be upon him) said: "You have drawn near to your enemy, and breaking the fast will give you strength." This was a concession, so some of us fasted and some broke the fast. Then we stopped at another halting place, and he (peace and blessings be upon him) said: "You will meet your enemy in the morning, and breaking the fast will give you strength, so break your fast." This was a firm command, so we broke our fast. Then he said: I saw us fasting on journeys with him (peace and blessings be upon him) after that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8149