Abu Dawud Book 2, Hadith Number 1225.

Chapter : When should a traveller observe complete prayer.

Narated By Imran ibn Husayn : I went on an expedition with the Apostle of Allah (PBUH), and I was present with him at the conquest. He stayed eighteen days in Mecca and prayed only two rak’ahs (at each time of prayer). And he said: You who live in the town must pray four; we are travellers.