Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1225
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.