Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) said: When the Prophet (peace and blessings be upon him) fell ill, he (peace and blessings be upon him) said: "Order Abu Bakr to lead the people in prayer." Sayyidah Aishah (may Allah be pleased with her) said: O Messenger of Allah! Abu Bakr is 'raqiq al-qalb' (tender-hearted). He (peace and blessings be upon him) said: "Tell Abu Bakr to lead the prayer." Sayyidah Aishah (may Allah be pleased with her) said the same thing again, and he (peace and blessings be upon him) said: "Tell Abu Bakr to lead the prayer; you are like 'sawahib Yusuf' (the companions of Yusuf)." So Sayyiduna Abu Bakr (may Allah be pleased with him) led the prayer during the lifetime of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16583