On the Recommendation to have company on a journey and the travellers should appoint someone as their amir whom they will obey

Riyad as-Salihin (The Meadows of the Righteous)

by Imam Nawawi

167. Chapter: On the Recommendation to have company on a journey and the travellers should appoint someone as their amir whom they will obey

958. Ibn 'Umar reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "If people knew what I know about travelling alone, no one would travel for a single night alone." [al-Bukhari]

959. 'Amr ibn Shu'ayb reported from his grandfather that the Messenger of Allah, may Allah bless him and grant him peace, said, "One rider is shaytan. Two riders are two shaytans. Three is a troop." [Abu Dawud, at-Tirmidhi, and an-Nasa'i]

960. Abu Sa'id and Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When three set out on a journey, they should appoint one of them as amir." [Abu Dawud]

961. Ibn 'Abbas reported that the Prophet, may Allah bless him and grant him peace, said, "The best [number] of companions is four, and the best of expeditions is 400 and the best of armies is 4000. 12,000 Muslims will not be defeated for lack of numbers." [Abu Dawud and at-Tirmidhi]