Imām al-Barbahārī (V died 329 AH) stated: “Know that Allah’s Messenger (H) said: “My ummah will divide into 73 sects, all of them will be in the Fire except for one, and that is the Jamā’ah.” It was said, “And who are they, O Allah’s Messenger?” He responded, “That which I and my Companions are upon today.” 
Shaikh Sālih Al-Fawzān explained:
“My ummah will divide into 73 sects, all of them will be in the Fire except for one, and that is the Jamā’ah.” Allah (C) has commanded us with unity upon the truth, as He stated:
وَاعْتَصِمُوا بِحَبْلِ اللَّهِ جَمِيعًا وَلَا تَفَرَّقُوا ۚ
“And hold fast to the Rope of Allah, altogether and be not divided.” (Āli ‘Imrān: 103) He also said:
إِنَّ الَّذِينَ فَرَّقُوا دِينَهُمْ وَكَانُوا شِيَعًا لَّسْتَ مِنْهُمْ فِي شَيْءٍ ۚ إِنَّمَا أَمْرُهُمْ إِلَى اللَّهِ ثُمَّ يُنَبِّئُهُم بِمَا كَانُوا يَفْعَلُونَ
“Indeed, those who have divided their religion and become sects, you O Prophet, are not associated with them in anything. Their affair is left only to Allah and He will inform them about what they used to do.” (Al-An’ām: 159) And He, the Most High, stated:
وَلَا تَكُونُوا كَالَّذِينَ تَفَرَّقُوا وَاخْتَلَفُوا مِن بَعْدِ مَا جَاءَهُمُ الْبَيِّنَاتُ ۚ وَأُولَٰئِكَ لَهُمْ عَذَابٌ عَظِيمٌ
“And do not be like the ones who became divided and differed after the clear proofs had come to them. And those will have a great punishment.” (Āli Imrān: 105) So Allah prohibited us from division and commanded us with unity and holding fast to the Book of Allah and the Sunnah of His Messenger, as He said:
وَأَنَّ هَٰذَا صِرَاطِي مُسْتَقِيمًا فَاتَّبِعُوهُ ۖ وَلَا تَتَّبِعُوا السُّبُلَ فَتَفَرَّقَ بِكُمْ عَن سَبِيلِهِ ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَتَّقُونَ
“And this is My path, which is straight, so follow it; and do not follow the other paths, for you will be separated from His Path. This has He instructed you that you may become righteous.” (Al-An’ām: 153) So differing and splitting by following ones desires is not allowed, and likewise the blind following of one’s father and forefathers, or to blindly follow the Jews and Christians. Differing is not permissible in the affairs of belief and in the foundations of the Religion; it is obligatory to have agreement and unity upon these matters.
As for differing that comes about in matters of the fiqh regulations then this can occur, but even here one must take only the sayings that are supported by the proofs. Allah (F) stated:
فَإِن تَنَازَعْتُمْ فِي شَيْءٍ فَرُدُّوهُ إِلَى اللَّهِ وَالرَّسُولِ إِن كُنتُمْ تُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ ۚ ذَٰلِكَ خَيْرٌ وَأَحْسَنُ تَأْوِيلًا
“And if you disagree over anything, refer it to Allah and the Messenger, if you should believe in Allah and the Last Day. That is the best way and best in result.” (An-Nisā: 59) The point is that differing in matters pertaining to belief (‘aqeedah) is not permitted because the ‘aqeedah is established by textual proofs alone and not open to juristic interpretation or opinion. As for the fiqh regulations and derived judgements and rulings, then those who are capable and worthy from the scholars will derive what they see to be most correct. And the scholars who have differed in their opinions did not [seek to] remain upon differing, instead they returned back to the Book of Allah and the Sunnah of His Messenger (H). And the one whose position was supported by the proofs, then it was followed, and his saying was accepted, and they would leave off their own opinions. This is the way of Ahlus-Sunnah wal-Jamā’ah and this is what the Messenger (O) directed us to. As for the common saying: “Leave the people alone, each to his own opinion because differing in the ummah is a mercy.” We say: this is sheer falsehood. Allah (C) stated:
وَلَا يَزَالُونَ مُخْتَلِفِينَ إِلَّا مَن رَّحِمَ رَبُّك
“But they will not cease to differ except those whom your Lord has shown mercy..” (Hūd: 118-119) So the His (E) saying “except whom your Lord has shown mercy” is a proof that that those who are shown mercy by Allah do not differ, and that differing is a punishment and not a mercy. Mercy is for those who do not differ – and if they do differ, they return to the Qur’an and Sunnah for resolution. They take what is authentic and they leave alone what is mistaken. This is the path of Ahlus-Sunnah wal-Jamā’ah. As for leaving each person to his own opinion in matters of Religion, then this is not way ordained for the Muslims. Rather that is the way of the people of whims and desires. They seek after opinions which agree with their desires, and they will abandon whatever opposes their desires even if it means abandonment of the statements of a great scholar whose saying they would [normally] take. Meaning: they will not accept the sayings of the great scholars unless they agree with their desires. So whatever opposes what they seek they reject it; and this proves they are followers of their desires since they take what accords with their desire; and what opposes it, they cast aside. Indeed there is no movement or might except with Allāh. This is what is being invited to in these times: in the newspapers, magazines, seminars, gatherings, at most conferences, the satellite channels where they publicise differing and say: “We are broadening the possibilities for the people.” With what are they broadening the possibilities? By leaving the Book and Sunnah and instead going to sayings which emanate from those who are not infallible? Those who can err sometimes and be correct sometimes? We say: It is these scholars who prevent us from taking their speech unless it conforms with the evidence – and they forbid us from taking their speech if it contradicts the proof. So it is a must that this affair is understood because in these times the people are put to trial by those who try to pull the wool over their eyes.
Imām al-Barbahārī (V died 329 AH) stated: “Know that Allah’s Messenger (H) said: “My ummah will divide into 73 sects, all of them will be in the Fire except for one…” This narration is authentic when its various chains and wordings are gathered together. The scholars have reported it and praised its authenticity; and the reality of the ummah further proves its truthfulness . The Messenger (H) stated that this ummah of Muhammad will divide into 73 sects – and these are the foundations of the sects, and there are of course more sects, but these 73 constitute the foundations of all of them. All of them will end up in the Fire except for one. So the seventy third sect is upon that which the Messenger (H) and his Companions (M) were upon – it is the sect saved (nājiyah) from the Fire. For this reason it is referred to as the Saved Sect (Firqatun-Nājiyah) and they are called Ahlus-Sunnah wal-Jamā’ah. And all those besides them are the opposers, those threatened with the Fire. From them are those who who will enter the Fire due to their unbelief, others due to their sins and others due to their disobedience – those who enter the Fire are not all the same. So it is not to be understood from this narration that all of the sects who enter the Fire are upon unbelief.
“…all of them will be in the Fire except for one, and that is the Jamā’ah.” The Jamā’ah refers to anyone who is upon the truth even if it is just one person. A gathering of large numbers in itself is not an indication that they are upon the truth. Allah (E) said:
وَإِن تُطِعْ أَكْثَرَ مَن فِي الْأَرْضِ يُضِلُّوكَ عَن سَبِيلِ اللَّهِ ۚ إِن يَتَّبِعُونَ إِلَّا الظَّنَّ وَإِنْ هُمْ إِلَّا يَخْرُصُونَ
“And if you obey most of those upon the earth, they will mislead you from the path of Allah. They follow not except assumption, and they are not but falsifying.” (Al-An’ām: 116) And He said:
وَمَا أَكْثَرُ النَّاسِ وَلَوْ حَرَصْتَ بِمُؤْمِنِينَ
“And most of mankind will not believe even if you desire it eagerly.” (Yoosuf: 103) And His saying:
وَمَا وَجَدْنَا لِأَكْثَرِهِم مِّنْ عَهْدٍ ۖ وَإِن وَجَدْنَا أَكْثَرَهُمْ لَفَاسِقِينَ
“And We did not find for most of them any covenant; but indeed, We found most of them defiantly disobedient.” (Al-A’rāf: 102) So attention is not given to numbers, rather attention is given to those who are upon the truth even if they are small in number in a given time or place. So even if there is just one person, he can be considered as the Jamā’ah.
It was said, “And who are they O Allah’s Messenger?” He responded, “That which I and my Companions are upon today.” So this is the correct path: whoever is upon that which the Messenger and his Companions were upon, he is the Jamā’ah.
See: It-hāf al-Qārī, Shaikh Al-Fawzān’s explanation of Sharhus-Sunnah of Imām al-Barbahārī, vol.1 pp. 419-423. Tirmidhī, no. 2641; Ibn Nasr al-Marwazī in As-Sunnah, no. 59; Al-Hākim in al-Mustadrak, 1/218; Al-Ājurrī in ash-Sharee’ah, no. 23; Al-Lālikā’ī in Sharh Usool al-I’tiqād, no. 147; Ibn Battah in al-Ibānah, no. 196; and many others. It has support from the hadīth of Anas reported by Tabarānī in Al-Awsat, no.7840; and in As-Saghīr, no. 724; It has been authenticated by a large body of scholars from the past and present, alhamdulillāh.