Musannaf Ibn Abi Shayba 3:5779

Abu Bakr narrated to us, he said: Sharik narrated to us, from Zubayd, from Abd al-Rahman ibn Abi Layla, from Umar, who said: The prayer of travel is two rak'ahs, complete, not shortened, upon the tongue of the Messenger of Allah, may Allah bless him and grant him peace.