Praise be to Allah.
The well known hadeeth about the ummah splitting into seventy-three sects is the hadeeth of Mu‘aawiyah ibn Abi Sufyaan (may Allah be pleased with him), who said: The Messenger of Allah (blessings and peace of Allah be upon him) stood up amongst us and said: “Those who came before you of the people of the Book split into seventy-two sects, and this ummah will split into seventy-three sects, seventy-two of which will be in the Fire and one in Paradise. That is the jamaa’ah (the main body of Muslims).”
Narrated by Abu Dawood (4597) and others; classed as saheeh by al-Haakim (1/128). In fact he said: It is an important hadeeth that highlights a major fundamental issue. It was also classed as saheeh by Ibn Taymiyah in Majmoo‘ al-Fataawa (3/345), ash-Shaatibi in al-I‘tisaam (1/430) and al-‘Iraqi in Takhreej al-Ihya’ (3/199).
The hadeeth was also narrated by at-Tirmidhi (2641) as follows: “My ummah will split into seventy-three sects, all of whom will be in the Fire except one sect.” They said: Who are they, O Messenger of Allah? He said: “[Those who follow] the path which I and my companions follow.” It was classed as hasan by Ibn al-‘Arabi in Ahkaam al-Qur’an (3/432), al-‘Iraqi in Takhreej al-Ihya’ (3/284) and al-Albaani in Saheeh at-Tirmidhi.
The words of the Prophet (blessings and peace of Allah be upon him), “My ummah will split into seventy-three sects” indicate that the differences that lead to splitting are differences concerning fundamentals and beliefs, not in minor issues and rulings of fiqh.
The words of the Prophet (blessings and peace of Allah be upon him) concerning the saved group, “That is the jamaa‘ah (the main body of Muslims)” or, according to another report “[Those who follow] the path which I and my companions follow” also confirm that. Those who differ concerning minor issues are not regarded as going beyond the bounds of the jamaa‘ah by doing so, or as going beyond the bounds of the teachings of the Prophet (blessings and peace of Allah be upon him) and his Companions. The Sahaabah differed concerning various minor issues, but that did not lead to their splitting and becoming divided. No one said that as a result of those (minor) differences they were included in the hadeeth about the sects; rather they were one group (jamaa‘ah), following one path and the same fundamentals of belief. The same may be said about the four Imams, the founders of the madhhabs that are followed, and other people of knowledge and virtue. They are the jamaa‘ah, the saved group, Ahl as-Sunnah. Whoever deviates from their basic principles and beliefs is the one who deserves to be included among the people of the sects and innovators who go astray.
Hence ash-Shaatibi (may Allah have mercy on him) said: These sects only became sects because of their differing from the saved group with regard to some fundamental matter of religion, or one of the basic principles of sharee‘ah, not with regard to a minor matter, because having a different view or an odd view on a minor matter does not lead to a difference that would result in splitting and division. Rather division occurs when there are differences concerning fundamental matters.
End quote from al-I‘tisaam (2/200)
Thus it is known that the four imams are innocent of division, as are their followers who adhere to the basic principles of Ahl as-Sunnah. As for those who differ from that and follow the ways of the Mu‘tazilah, Shi‘ah, Murji’ah or other sects and followers of whims and desires, this is the one who has gone against Ahl as-Sunnah and is included among the blameworthy sects.
For more information, please see the answer to question no. 90112
And Allah knows best.