Abu Salamah narrates from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The Jews split into seventy-one or seventy-two (71 or 72) sects, the Christians split into seventy-one or seventy-two sects, and my ummah will split into seventy-three sects."
Abu Sulayman al-Khattabi (may Allah have mercy on him) said: His (peace and blessings be upon him) saying, 'My ummah will split into seventy-three sects,' means that all these sects will not be outside the religion; rather, they will be counted within his (peace and blessings be upon him) ummah. It also shows that one who interprets (muta'awwil) does not leave the religion, even if he errs in his interpretation.
The Shaykh (may Allah have mercy on him) said: Whoever calls a Muslim a disbeliever outright does not leave the religion because of calling him a disbeliever, as in the hadith of Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both): a man left the prayer behind Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him), and he called him a hypocrite. When the Prophet (peace and blessings be upon him) learned of it, he (peace and blessings be upon him) ordered Sayyiduna Mu'adh (may Allah be pleased with him) to lighten the prayer and said: "O Mu'adh! Are you one who causes trials (fitnah)?" meaning: do not lead such long prayers.
When Sayyiduna Hatib bin Abi Balta'ah (may Allah be pleased with him) tried to inform the Quraysh of the Prophet's (peace and blessings be upon him) advance against them, Sayyiduna Umar (may Allah be pleased with him) said: "O Messenger of Allah (peace and blessings be upon him)! If you permit, I will strike off the neck of this hypocrite." He (peace and blessings be upon him) said: "He took part in Badr." The Prophet (peace and blessings be upon him) did not pronounce takfir on Sayyiduna Umar (may Allah be pleased with him) for calling him a hypocrite, because this was an outward sign of hypocrisy; the one who becomes a disbeliever is he who declares a Muslim a disbeliever without any interpretation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20901