Musnad Ahmad 27:1603

Yunus narrated to us, Hammad—meaning Ibn Zayd—narrated to us, Bishr ibn Harb narrated, saying: I asked Ibn Umar, "How is the prayer of the traveler, O Abu Abd al-Rahman?" He said, "Either you follow the Sunnah of your Prophet, may Allah bless him and grant him peace, and I will inform you, or you do not follow the Sunnah of your Prophet, and I will not inform you." He said: We said, "The best of Sunnahs is the Sunnah of our Prophet, may Allah bless him and grant him peace, O Abu Abd al-Rahman." He said: "When the Messenger of Allah, may Allah bless him and grant him peace, would leave this city, he would not exceed two rak'ahs until he returned to it."