Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: "When you are three people on a journey, appoint one of yourselves as a leader over you. That will be a leader whom the Messenger of Allah appointed (or: that will be a leader about whom the Messenger of Allah gave the command)."
Al-Bazzar recorded it, and its narrators are the narrators of the Sahih, except Ammar bin Khalid, who is thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9305