Abu Dawud Book 2, Hadith Number 1219.

Narated By Hafs b. ‘Asim : I accompanied Ibn ‘Umar on the way (on a journey). He led us in two rak’ahs of (the noon) prayer. Then he proceeded and saw some people standing. He asked: What are they doing? I replied: They are glorifying Allah (i.e. offering supererogatory prayer). He said: If I had offered the supererogatory prayer (while travelling), I would have completed prayer, my cousin. I accompanied the Apostle of Allah (PBUH) during the journey, he did not pray more than two rak’ahs until his death. I also accompanied Abu Bakr, and he prayed two rak’ahs and nothing more until he died. I also accompanied ‘Umar and he prayed two rak’ahs and nothing more until he died. I also accompanied ‘Uthman and prayed two rak’ahs and nothing more until he died. Allah, the exalted said: “Certainly you have in the Messenger of Allah an excellent exemplar” (xxxiii.).