To Talhah and az-Zubayr (through `Imran ibn al-Husayn al-Khuza`i) 1 
Abu Ja`far al-Iskafi has mentioned this in his "Kitab al-maqamat" on the excellent qualities (manaqib) of Amir al-mu'minin (peace be upon him).
Now, both of you know, although you conceal it, that I did not approach the people till they approached me, and I did not ask them to swear allegiance to me till they themselves swore allegiance to me, and both of you were among those who approached me and swore me allegiance. Certainly, the common people did not swear me allegiance under any force put on them or for any money given to them. 
If you two swore allegiance to me obediently, come back and offer repentance to Allah soon, but if you swore allegiance to me reluctantly, you have certainly given me cause for action, by showing your obedience and concealing your disobedience. 2 By my life, you were not more entitled than other Muhajirun to conceal and hide the matter. Your refusing allegiance before entering into it would have been easier than getting out of it after having accepted it.
You have indicated that I killed `Uthman; then let someone from among the people of Medina who supported neither me nor you decide the matter between me and you. Then one of us shall face (the command of law) according to (their) involvement. You should give up your way now, O you two elderly men, when the great question before you is only one of shame, before you face the question of shame coupled with the Hell-fire; and that is an end to the matter.