Sahih Bukhari Volume 001, Book 011, Hadith Number 651.
Narated By 'Urwa's father : 'Aisha said, "Allah's Apostle ordered Abu Bakr to lead the people in the prayer during his illness and so he led them in prayer." 'Urwa, a sub narrator, added, "Allah's Apostle felt a bit relieved and came out and Abu Bakr was leading the people. When Abu Bakr saw the Prophet he retreated but the Prophet beckoned him to remain there. Allah's Apostle sat beside Abu Bakr. Abu Bakr was following the prayer of Allah's Apostle and the people were following the prayer of Abu Bakr."