Once during his caliphate, Ali ibn Abi Taalib stood up before the people and delivered the following sermon:
"O people, who is the bravest of all people?" Those that were present answered, "You are, O Leader of the Believers." Ali said, "Lo, I never entered into a duel with another man except that I came out victorious over him. And yet, the bravest of all people was none other than Abu Bakr Ra. We once build a command centre for RasulAllah SAW, and we said, "Who will stay with RasulAllah SAW, so as to prevent the polytheists from attacking him? By Allah, not a single one of us approached (to volunteer for the job) except for Abu Bakr Ra, who, with his sword unsheathed, stood over the head of RasulAllah SAW. Abu Bakr Ra then fought against anyone who tired to attack.
I remember the time when I saw the Quraish opposing the Prophet SAW and physically shaking him violently, and all the while they were saying to him, "Have you taken our Gods and made them into one God!" By Allah, not a single one of us stepped forward (to defend him) except for Abu Bakr Ra, who struggled against and struck one man and pushed violently another; all the while he was saying (to them), "Woe upon you, are you fighting against a man because he says: My Lord is Allah?" Ali Ra then lifted the robe he was wearing, and he began to cry until his beard became soaked in tears. He then continued, "I ask you by Allah, was the believer from the family of the Pharaoh better or Abu Bakr?" The attendees of the gathers then began to cry along with Ali Ra, who said, "By Allah, Abu Bakr for an hour is better than the believer of the family of the Pharaoh even if clones of him were to fill the entire earth: the latter kept his faith a secret while the former openly proclaimed his faith"
Al-Bidaayah Wan-Nihaayah (3/271, 272)