When Hazrat Abu Bakr (radhiyallahu ‘anhu) was in his final illness, he dictated the following letter to Hazrat ‘Uthmaan (radhiyallahu ‘anhu) in which he appointed Hazrat ‘Umar (radhiyallahu ‘anhu) as the Khalifah after him:
Bismillahir Rahmaanir Raheem
This is the instruction of Abu Bakr, the son of Abu Quhaafah, at the very end of his life in this world, while exiting the world and entering the life of the Hereafter, and he is close to reaching the point (of the throes of death, wherein the signs of the Hereafter become clear to a person and) where a kaafir will wish that he had brought Imaan, a sinner realizes and gains full conviction of his condition, and a liar will wish to become truthful.
I have appointed ‘Umar, the son of Khattaab, as the Khalifah over you. You should thus listen to him and express complete obedience to him, for indeed I have not fallen short in fulfilling my obligation to Allah, His Rasul (sallallahu ‘alaihi wasallam), His deen, and the right I owe to myself and to you, in appointing the best person over you who is most worthy of the Khilaafah from the people.
If he rules with justice then that is what I expect of him and that is what I know of him, and if he changes his ways ( after I leave this world, and becomes evil and oppressive) then each man will have to bear the sin that he earns. It is only goodness that I intended in appointing him, and I do not have knowledge of the unseen.
وَسَيَعْلَمُ ٱلَّذِينَ ظَلَمُوٓا۟ أَىَّ مُنقَلَبٍ يَنقَلِبُونَ
The wrongdoers will soon come to know to what end they will return
Was salaamu ‘alaikum warahmatullah
To be continued Insha Allah
(Hayaatus Sahaabah 2/ 31)