Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 916
Narated By Sahl ibn al-Hanzaliyyah : The iqamah for the morning prayer was pronounced and the Apostle of Allah (pbuh) began to offer prayer while he was looking at the mountain-pass.
(AbuDawud elaborated that the Prophet had sent a horseman to the mountain-pass at night in order to keep watch.)