Abu al-Bakhtari (may Allah have mercy on him) says that Ubaydah (may Allah have mercy on him) said: When a man sets out on a journey having already fasted part of Ramadan, let him fast the rest; and he recited this verse: "So whoever of you witnesses the month, let him fast it." [Surah al-Baqarah: 185]
Abu al-Bakhtari narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), and he had more understanding than us: "Whoever wishes may fast, and whoever wishes may break the fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8177