Abu Dawud Book 2, Hadith Number 1229.
Chapter : When should a traveller observe complete prayer.
Narated By Anas b. Malik : We went out from Medina to Mecca with the Apostle of Allah (PBUH) and he prayed two rak’ahs (at each time of prayer) till we returned to Medina. We (the people) said: Did you stay there for some time? He replied: We stayed there ten days.