Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 206
Narrated Ibn 'Umar :
I accompanied Allah's Apostle and he never offered more than two Rakat during the journey. Abu Bakr, 'Umar and 'Uthman used to do the same.