Musannaf Ibn Abi Shayba 36:564

Waki’ narrated to us, from al-A’mash, from Salim, from Ubayd Allah ibn Subay’, who said: I heard Ali saying: "This [pointing to his beard] will be dyed by this [pointing to his head]. So what is being awaited regarding the most wretched one?" They said: "Inform us of him so we may kill him." He said: "Then, by Allah, you would be killing someone other than my killer." They said: "Will you not appoint a successor?" He said: "No, but I will leave you to that to which the Messenger of Allah (peace and blessings of Allah be upon him) left you." They said: "What will you say to your Lord when you meet Him?" He said: "I will say: O Allah, You left me among them, then You took me to Yourself while You are among them; so if You will, You will rectify them, and if You will, You will corrupt them."