Musannaf Ibn Abi Shayba 37:316

Waki’ narrated to us, from al-A’mash, from Salim ibn Abi al-Ja’d, from ‘Abdullah ibn Subay’ (Sabu’), who said: ‘Ali delivered a sermon to us and said: “This will surely dye this,” meaning his beard from his head. They said: “Inform us of him so we may kill him.” He said: “Then, by Allah, you would kill someone other than my killer because of me.” They said: “Then appoint a successor over us.” He said: “No, but I will leave you to that which the Messenger of Allah left you to.” They said: “What will you say to your Lord when you meet Him?” He said: “I will say: ‘O Allah, I was among them, then You took me to Yourself while You were among them; so if You will, You will rectify them, and if You will, You will corrupt them.’”