Musannaf Ibn Abi Shayba 3:1080

Waki’ narrated to us, from Thawr al-Shami, from Muhajir ibn Habib, from Abu Salamah ibn Abd al-Rahman, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If three Muslims go out on a journey, let the one among them who is most proficient in the Book of Allah lead them in prayer, even if he is the youngest of them. If he leads them in prayer, he is their leader, and that is a leader whom the Messenger of Allah (peace and blessings of Allah be upon him) appointed."