It is narrated from Jabir (may Allah be pleased with him) that he reports: The Messenger of Allah (peace and blessings be upon him) set out towards Makkah in the year of the conquest of Makkah during Ramadan, and he ordered the people to break their fast and said: "Strengthen yourselves against your enemy." It was said to him that some people refused to break their fast. When he (the Prophet) kept the fast, he called for a cup of water and drank from it. Then he narrated the rest of the hadith at length.
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 622
Hadith Takhrijأخرجه مسلم، الصيام، باب جواز الصوم والفطر في شهر رمضان للمسافر في غير معصية (1114).