Sahih Bukhari Volume 004, Book 055, Hadith Number 599.
Narated By Abu Musa : When the Prophet fell ill, he said, "Order Abu Bakr to lead the people in prayer." 'Aisha said, "Abu Bakr is a soft-hearted person. The Prophet gave the same order again and she again gave the same reply. He again said, "Order Abu Bakr (to lead the prayer)! You are (like) the female companions of Joseph." Consequently Abu Bakr led the people in prayer in the life-time of the Prophet.