There are many verses and Hadīths that urge us to adhere to the Sunnah and hold onto it and warn us against departure from it and forbid us from disunity and conflict. Allah Almighty says: {Indeed, in the Messenger of Allah you have an excellent example for those who look forward to Allah and the Last Day, and remember Allah much.} [Surat al-Ahzāb: 21] He also says: {Those who have made divisions in their religion and turned into factions, you have nothing to do with them.} [Surat al-An‘ām: 159] Allah Almighty also says: {He has prescribed for you [believers] the same religion which He enjoined upon Noah, and that which We have revealed to you [O Prophet] and that which We enjoined upon Abraham, Moses, and Jesus: "You should uphold the religion and make no divisions therein."} [Surat ash-Shūra: 13]
Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Behold! The best speech is the Book of Allah, and the best guidance is that of Muhammad. The worst things are the newly-introduced ones, and every newly- introduced thing is misguidance.” [Narrated by Muslim (867), An-Nasā’i (1578), Abu Dāwūd (2954), and Ibn Mājah (45)]
Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said: "All of my Ummah will enter Paradise except those who refuse." They said: "Who would refuse, O Messenger of Allah?'' He replied: "Whoever obeys me will enter Paradise, and whoever disobeys me has refused." [Narrated by Al-Bukhāri (7280)]
Al-‘Irbād ibn Sāriyah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) gave us an eloquent admonition one day after the Fajr prayer which caused the eyes to shed tears and the hearts to become afraid. Someone said: "This is the admonition of someone who will soon part. What do you charge us with doing, O Messenger of Allah?" He said: "I advise you to fear Allah and to hear and obey, even if an Ethiopian slave were placed in charge of you. Those of you who live after me will see much differing. Beware of newly-invented matters, for they are misguidance. Those of you who live to reach those matters should hold fast to my Sunnah and that of the rightly-guided Caliphs. Bite onto it with your molar teeth." [Narrated by Abu Dāwūd (4607), At-Tirmidhi (2676), Ibn Mājah (42), Ahmad (17416), and Ad-Dārimi (96); classified as Sahīh (Authentic) by At-Tirmidhi]
Al-Bukhāri said in his Sahīh, in the Book of Holding Onto the Qur’an and the Sunnah, Chapter on Adhering to the Prophet's Sunnahs and the Verse that Reads: {And make us leaders for the righteous} [Surat al-Furqān: 74]: Leaders who follow the example of those before us and whose example is followed by those who come after us. Ibn ‘Awn said: I love three things for myself and my brothers: This Sunnah - that they learn and ask about it; the Qur’an - that they ponder and ask about it; and leaving people except when it comes to goodness." [Sahīh Al-Bukhāri (8/139)