Short answer
Abu Bakr died of an illness with fever in Madinah on 22 Jumada al-Akhirah 13 AH (August 634 CE), aged 63, after about two years and three months as caliph. Before he died he named Umar as his successor, and he was buried beside the Prophet ﷺ.
The mainstream Sunni account is that Abu Bakr fell ill with a fever for about fifteen days and died at 63, the same age as the Prophet ﷺ. Some weaker reports speak of poison; most historians give illness.
The four schools agree on this.
Explanation
According to the best-known account, Abu Bakr bathed on a cold day, caught a fever and was ill for about fifteen days. During his illness he consulted senior Companions and then appointed Umar ibn al-Khattab to succeed him, writing it down through Uthman. Some reports say he had been given poison a year earlier, but historians treat that as a weaker report.
Aisha tells in Sahih al-Bukhari that during his last illness he asked her on which day the Prophet ﷺ had died. She said Monday. He asked what day it was, and it was Monday; he hoped to die before nightfall. He died on the eve of Tuesday and was buried before morning. He also asked to be shrouded in his own worn clothes, saying the living have more need of new cloth than the dead.
He was 63, the same age as the Prophet ﷺ. He was buried beside him in Aisha's room, his head level with his friend's shoulders. Umar was later buried beside them.
His caliphate was short but decisive: he held the Muslims together after the Prophet's ﷺ death, fought the Ridda wars, had the Quran collected and began the campaigns into Iraq and Syria. Read his life on Abu Bakr al-Siddiq and How was Abu Bakr chosen as the first caliph?
The evidence
Sahih al-Bukhari 1387
Sahih... في أي يوم توفي رسول الله صلى الله عليه وسلم ...
"On which day did the Messenger of Allah die?"
His question to Aisha in his last illness; he died on the eve of Tuesday.
Sahih Muslim 2348
Sahih... وأبو بكر وهو ابن ثلاث وستين ...
"And Abu Bakr died at sixty-three."
His age at death.
Where the ruling is recorded
- al-Bidaya wa al-Nihaya (Ibn Kathir), Events of the year 13 AH, the death of Abu Bakr. The illness, the appointment of Umar and the date.
- al-Isaba fi Tamyiz al-Sahaba (Ibn Hajar), Entry on Abu Bakr al-Siddiq. His death, age and burial.
- Fath al-Bari (Ibn Hajar), Commentary on Kitab al-Jana'iz, chapter on dying on Monday. Explains Abu Bakr's wish and the hadith of Aisha.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.