Musnad Ahmad 5:514

Waki’ narrated to us, al-A’mash narrated to us, from Salim ibn Abi al-Ja’d, from ‘Abdullah ibn Sabu’, who said: I heard ‘Ali saying: "This [pointing to his beard] will be dyed by this [pointing to his head], so what is the most wretched of people waiting for regarding me?" They said: "O Commander of the Faithful, inform us of him so we may cut off his lineage." He said: "Then, by Allah, you would be killing someone other than my killer." They said: "Then appoint a successor over us." He said: "No, but I will leave you to that 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 come to Him?" — and Waki’ said once: "When you meet Him?" — He said: "I will say: O Allah, You left me among them for as long as You willed, 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."