Musnad Ahmad 27:1103

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 we 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 see if we are at Dhu al-Majaz?" He said: "And what is Dhu al-Majaz?" I said: "A place where we gather, and sell in it, 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, peace and blessings of Allah be upon him, before my very eyes, praying them two rak'ahs, two rak'ahs." Then he recited this verse: "There has certainly been for you in the Messenger of Allah an excellent pattern" [Al-Ahzab: 21], until he finished the verse.