Mishkat al-Masabih 19 Jihad كتاب الجهاد 123

وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيُّ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِذَا كَانَ ثَلَاثَةٌ فِي سَفَرٍ فَلْيُؤَمِّرُوا أحدهم» . رَوَاهُ أَبُو دَاوُد

Abu Sa’id al-Khudri reported God’s Messenger as saying, “When three are on a journey they should appoint one of them as their commander.” Abu Dawud transmitted it.