Musnad Ahmad 27:592

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Khubayb—meaning Ibn ‘Abd al-Rahman—from Hafs ibn ‘Asim, from Ibn ‘Umar, who said: "We went out with the Messenger of Allah (may Allah bless him and grant him peace), and he used to perform the prayer of travel—meaning two rak‘ahs—and [he did so] with Abu Bakr, and ‘Umar, and ‘Uthman for six years of his caliphate. Then he prayed four."