Whoever leaves the community by a handspan has removed the noose of Islam from his neck until he returns

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever separates from the Jama'ah (community) even by a handspan has thrown off the collar of Islam from his neck, until he returns (to the Jama'ah)." And he said: "Whoever dies while there is no Imam of the (Muslim) Jama'ah over him, his death is a death of Jahiliyyah (pre-Islamic ignorance)." And the Messenger of Allah (peace and blessings be upon him) delivered a sermon and said: "O people! I will be your forerunner at the Pool (the one who arrives first to receive you). Its extent is as the distance between Kufah and the Black Stone, and its vessels are as numerous as the stars. And I saw some people of my Ummah: when they came near me, a man appeared and took them away from me. Then another group came, and he did the same with them. So none of them succeeded (in reaching the Pool) except a few, like animals strayed from the herd (very few)." Sayyiduna Abu Bakr (may Allah be pleased with him) said: O Prophet of Allah! Am I among them? He (peace and blessings be upon him) said: "No, rather they are people who will come after you and will turn back on their heels (from the religion)."
This hadith is sahih (authentic) according to the conditions of the two Shaykhs (Bukhari and Muslim), and Hajjaj bin Muhammad also narrated it from al-Layth, but they did not record it.

It is narrated from Sayyiduna Anas (may Allah be pleased with him) that he said: I went to Ubaydullah bin Ziyad while they (he and his companions) were arguing back and forth about the Pool of al-Kawthar. He said: Anas (may Allah be pleased with him) has come to you. Then he asked: O Anas! What do you say about the Pool? I said: I never imagined I would live long enough to see people like you doubting and arguing about the Pool. I have left behind me old women, not one of whom prays a prayer without asking her Lord to bring her to the Pool of Muhammad (peace and blessings be upon him).
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Anas (may Allah be pleased with him) that he said: I went to Ubaydullah bin Ziyad while they were discussing the Pool; then he narrated the same as the previous narration.
There is also a sahih shahid (supporting narration) of it from Humayd on the condition of the two Shaykhs (Bukhari and Muslim).