Abu Dawud Book 2, Hadith Number 916.
Chapter : Not known.
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.)