Sayyiduna Hamzah bin Amr al-Aslami (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him): O Messenger of Allah, I am a man who fasts continuously; may I fast on a journey? He (peace and blessings be upon him) said: "Fast if you wish, and break your fast if you wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8157