Sunan Abi Dawud 916
سنن أبي داود#916SahihNarrated Sahl ibn al-Hanzaliyyah: The iqamah for the morning prayer was pronounced and the Messenger of Allah (ﷺ) 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiIsnaad Hasan
In-book reference: 2:527