I have heard Messenger of Allaah (sallallahu alayhi wa sallam) say, ‘Whenever people see evil and fail to change it, the time is close when Allaah shall engulf them all in great punishment.’
—  Abu Bakr as-Siddeeq (radiAllaahu anhu) [Musnad Ahmad]
When Umar ibn Al-Khattab (radiAllaahu anhu) was placed on his bed, the people gathered around him, praying and invoking blessings upon him before (the bier) was lifted up, and I was among them. No one alarmed me except a man who crowded against me and seized me by the shoulder. I turned and saw that it was Ali ibn Abu Talib (radiAllaah anhu). He prayed for mercy for Umar, then he said: “you have not left behind anyone who is more beloved to me to meet Allaah with the like of his deeds than yourself. By Allaah, I think that Allaah will most certainly unite you with your two companions, and that is because I often heard the Messenger of Allaaah (sallallahu alayhi wa sallam) saying: ‘Abu Bakr, Umar and I went (somewhere); Abu Bakr, Umar and I came in; Abu Bakr, Umar and I went out.’ So I think that Allaah will most certainly join you to your two companions.”
—   Ibn Abbas (radiAllaahu anhu) [Sahih Bukhari; Ibn Majah]
You were the first of the people to accept Islaam, and the most sincere of them in Eeman (faith). You were the best of them as a companion, and the most like the Messenger (salallaahu ‘alayhi wa sallam) in both guidance and humility, and the most generous of them to him. You succeeded him in the Religion well when the people left Islaam, and you held firm to the methodology of the Messenger (salallaahu ‘alayhi wa sallam). You were like a mountain that was not moved by the storm nor crushed by the quake. You are modest with yourself, and great with Allaah. And the closest people to you are the most obedient to Allaah, and the most pious. May Allaah make you join the company of his Messenger, and may He not deprive us of your reward. And may He not lead us astray after you.
—  Ali ibn Abi Talib (radiAllaahu anhu) describing Abu Bakr (radiAllaahu anhu) [Tarikh al-Islam by Adh Dhahabi]

Abu Bakr (radiAllaahu anhu) once asked, “O Messenger of Allaah (sallallahu alayhi wa sallam)! When we will be punished for every sin, who can keep himself in good stead after the verse:

Whoever does a wrong will be recompensed for it [Qur’aan (4:123)]

Messenger of Allaah (sallallahu alayhi wa sallam) said, ”May Allaah forgive you, O Abu Bakr! Do you not fall ill? Do you not get tired? Do you never feel depressed? Do you never suffer hardship? Do calamities never befall you?” 

"These things do happen,"  Abu Bakr replied.

Messenger of Allaah (sallallahu alayhi wa sallam) said, “This is the retribution (for your sins) in this world.”
 [Musnad Ahmad]

Imam Ibn Qayyim al-Jauziyyah:

"He (I.e Abu Bakr) protected the Messenger with everything at his disposal. He accompanied the Prophet (‘Alayhi salaatu wa salaam) during his life and death. How could anyone deny his virtues? To do so would be to try to cover the sun in the middle of the day. He is the best among the Companions, and whoever loves him is a believer."

(al-Fawaa’id, p.114)