The Prophet (Peace be upon him) said, “Allah sent me (as a Prophet) to you (people) but you said (to me), ‘You are telling a lie, ‘While Abu Bakr said, ‘He has said the truth.

Sahih al-Bukhari 3661

And another Narration “While addressing the people, Ali (may Allah be pleased with him) once asked, “O people! Who is the most courageous person?”

“You are, O Ameerul Mu’mineen=Prince of Believers,” the people submitted. Ali (may Allah be pleased with him) then said,

“Although I have defeated everyone who has confronted me, the most courageous person is Abu Bakr (may Allah be pleased with him). We had constructed a shed for Messenger of Allah peace be upon him (during the Battle of Badr) and then asked who would remain with Prophet peace be upon him so that the Mushrikin=Polytheists do not attack him.

By Allah! Whenever a Mushrik even drew close to us Abu Bakr (may Allah be pleased with him) was there with his sword drawn near the head side of Prophet peace be upon him. He attacked anyone who dared attack Messenger of Allah peace be upon him. He was certainly the bravest of people.”

Ali (may Allah be pleased with him) continues, “I have seen the Quraysh grab hold of Prophet peace be upon him with one person treating him angrily and another shaking him while they said to him, ‘Do you make all the gods into one?!’

 By Allah! None of us dared go close to Messenger of Allah peace be upon him (for fear of being beaten) besides Abu Bakr (may Allah be pleased with him). He would hit one person, wrestle with another and shake someone else as he said, ‘Shame on you people! Will you kill a man for saying, ‘Allah is my Rabb’?”‘

Ali (may Allah be pleased with him) then lifted the shawl he was wearing and wept until his beard became wet. He then said, “I ask you to swear by Allah whether the Momin from the court of Firhoun was better or Abu Bakr (may Allah be pleased with him) When everyone remained silent, Ali (may Allah be pleased with him) said, “By Allah! A moment of the life of Abu Bakr (may Allah be pleased with him) is better than the earth full of people like the Mu’min from the court of Fir’oun. While the Mu’min from the court of Fir’oun concealed his Iman=faith, Abu Bakr (may Allah be pleased with him) made his Iman=faith public.”

[Bazzaar as quoted in Al Bidaaya wan Nihaaya (Vo1.3 Pg.271). Haythami (Vo1.9 Pg.47) has commented on the chain of narrators reliable]

And further, The Prophet (Peace be upon him) said, “If I were to take a Khalil, I would have taken Abu Bakr, but he is my brother and my companion (in Islam) .

Sahih al-Bukhari 3656

May Allah be pleased with him.

Reference: Abu Bakr