Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I saw the Messenger of Allah (peace and blessings be upon him) gesturing with his palm like this, as if he were measuring something by the span of his hand, and he was saying: "Whoever separates from the community of the Muslims by even a handspan has thrown the noose of Islam off his neck. And the opponents will hold their banners from behind their backs on the Day of Resurrection. And whoever dies without an imam of the community dies the death of Jahiliyyah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13547