Musnad Abu Ya'la al-Mawsili 150:294

Zakariya ibn Yahya al-Raqashi narrated to us, Yusuf ibn Khalid narrated to us, Musa al-Makki narrated to us, from Musa ibn Talhah, from Aisha bint Sa'd, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let Abu Bakr lead the people in prayer." They said: "O Messenger of Allah, if only you would command someone else to lead the prayer." He said: "It is not fitting for my nation that an imam should lead them while Abu Bakr is among them."