It is narrated from Sayyiduna Hudhayfah bin al-Yaman (may Allah be pleased with him): People used to ask the Messenger of Allah (peace and blessings be upon him) about good, and I used to ask him (peace and blessings be upon him) about evil, for fear that it might overtake me. I said: "O Messenger of Allah, we were in ignorance (jahiliyyah) and evil, and then Allah brought us this good (Islam). Will there be any evil after this good?" He (peace and blessings be upon him) said: "Yes, and there will be dakhan (smoke, impurity) in it." I asked: "What is its impurity?" He said: "People who follow a way other than my guidance; some of their ways will be good and some bad." I asked: "Will there be any evil after that good?" He said: "Yes, callers standing at the gates of Hell; whoever responds to their call, they will throw him into it." I said: "O Messenger of Allah, describe them to us." He said: "They will be from our own people and will speak our own language." I said: "What do you command me if I live to see that time?" He said: "Hold fast to the community (jama'ah) of the Muslims and their imam." I said: "And if they have neither a community nor an imam?" He said: "Then separate yourself from all those sects, even if you have to bite on the root of a tree, until death comes to you while you are in that state." This hadith is found in this way in the "Sahihayn" (Bukhari and Muslim), and it is also narrated in abridged form through the narration of Imam al-Zuhri. I have mentioned it here in the Book of Knowledge because I did not find with the two Shaykhs a better proof than this that consensus (ijma') is a binding proof.