Musnad Ahmad 27:1300

Yunus ibn Muhammad narrated to us, Al-Harith ibn Ubayd narrated to us, Bishr ibn Harb narrated to us, he said: I asked Abdullah ibn Umar, he said: I said, "What do you say regarding fasting while traveling?" He said, "Will you accept it if I tell you?" I said, "Yes." He said, "When the Messenger of Allah (peace and blessings of Allah be upon him) would depart from this city, he would shorten the prayer and would not fast until he returned to it."