Prayer (Kitab Al-Salat): Details of Commencing Prayer
Dawud :: Book 3 : Hadith 916
Narrated Sahl ibn al-Hanzaliyyah:
The iqamah for the morning prayer was pronounced and the Apostle of Allah (peace_be_upon_him) 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.)