Musnad Ahmad 27:1117

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Isma'il, I heard Hakim al-Hadhdha', I heard Ibn 'Umar, he was asked about the prayer while traveling? He said: "Two rak'ahs, the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace."