Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

"The Prophet became sick and when his disease became aggravated, he said, "Tell Abu Bakr to lead the prayer." 'Aisha said, "He is a soft-hearted man and would not be able to lead the prayer in your place." The Prophet said again, "Tell Abu Bakr to lead the people in prayer." She repeated the same reply but he said, "Tell Abu Bakr to lead the people in prayer. You are the companions of Joseph." So the messenger went to Abu Bakr (with that order) and he led the people in prayer in the lifetime of the Prophet.