Musnad Ahmad 27:1964

Muhammad ibn Bakr narrated to us, Yahya ibn Qays al-Ma’aribi informed us, Thumamah ibn Shurahil narrated to us, he said: I went out to Ibn Umar and said: "What is the prayer of the traveler?" He said: "Two rak'ahs, two rak'ahs, except for the Maghrib prayer, which is three." I said: "What do you think if we are at Dhu al-Majaz?" He said: "What is Dhu al-Majaz?" I said: "A place where we gather, sell, and stay for twenty nights or fifteen nights." He said: "O man, I was in Azerbaijan—I do not know if he said four months or two months—and I saw them praying it two rak'ahs, two rak'ahs. And I saw the Prophet of Allah, may Allah bless him and grant him peace, with my own eyes praying it two rak'ahs." Then he recited to me this verse: "There has certainly been for you in the Messenger of Allah an excellent pattern" [Al-Ahzab: 21].