Sahl bin al-Hanzaliyyah (may Allah be pleased with him) narrates that the iqamah for the morning prayer had been called, and the Messenger of Allah (peace and blessings be upon him) was praying while also looking towards the mountain pass. Abu Dawud (may Allah have mercy on him) states that he (peace and blessings be upon him) had sent a horseman to the mountain pass during the night to keep watch (whom he was looking out for).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3868