Musnad Ahmad 790:144

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Ali ibn Zayd, who said: I heard Abu Nadrah say: ‘Imran ibn Husayn passed by our mosque, so I stood up to him and took hold of his bridle, and I asked him about prayer while traveling. He said: "We went out with the Messenger of Allah (may Allah bless him and grant him peace) for Hajj, and he would pray two rak‘ahs until he passed away. Abu Bakr would pray two rak‘ahs until he passed away. ‘Umar would pray two rak‘ahs until he passed away. And ‘Uthman [did so] for six or eight years, then he completed the prayer at Mina as four."