Chapter 6: The Book on Traveling
Abu An-Nadrah said: "Imran bin Husain was asked about the traveler' s prayer. He said: 'I performed Hajj with the Messenger of Allah, and he prayed two Rak' ah. I performed Hajj with Abu Bakr, and he prayed two Rak' ah and with Umar, and he prayed two Rak;ah; and with Uthman for six years of his Khilafah, or eight years, and he prayed two Rak' ah."
Jami at-Tirmidhi 545