Musnad Ahmad 21:22

Yahya ibn Adam narrated to us, Qays narrated to us, Abdullah ibn Abi al-Safar narrated to us, from Arqam ibn Sharhabil, from Ibn Abbas, from al-Abbas ibn Abd al-Muttalib, that the Messenger of Allah (may Allah bless him and grant him peace) said during his illness: "Command Abu Bakr to lead the people in prayer." So Abu Bakr went out and pronounced the takbir. Then the Prophet (may Allah bless him and grant him peace) felt a sense of relief, so he went out being supported between two men. When Abu Bakr saw him, he stepped back, but the Prophet (may Allah bless him and grant him peace) signaled to him: "Stay where you are." Then the Messenger of Allah (may Allah bless him and grant him peace) sat beside Abu Bakr, and he recited from the place in the surah that Abu Bakr (may Allah be pleased with him) had reached.