Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"Sahl bin Sad As-Sa'idi used to give preference to the young to lead his people in prayer. It was said to him: "Do you do that, when you have such seniority (in Islam)?" He said: "I heard the Messenger of Allah (saw) say: 'The Imam is responsible. If he does well, then he will have the reward and so will they, but if he does badly, then that will be counted against him but not against them.'" Sahih