Holding to the Prophet's way and leaving innovation in the religion.
4 hadith
On the authority of the mother of the faithful, Aisha (ra), who said: The Messenger of Allah (ﷺ) said, “He who innovates something in this matter of ours (i.e., Islam) that is not of it will have it rejected (by Allah).” In another version in Muslim it reads: “He who does an act which we have not commanded, will have it rejected (by Allah).”
On the authority of Abu Hurayrah (ra): I heard the Messenger of Allah (ﷺ) say, “What I have forbidden for you, avoid. What I have ordered you [to do], do as much of it as you can. For verily, it was only the excessive questioning and their disagreeing with their Prophets that destroyed [the nations] who were before you.”
On the authority of Abu Najeeh al-’Irbaad ibn Saariyah (may Allah be pleased with him) who said: The Messenger of Allah (peace and blessings of Allah be upon him) gave us a sermon by which our hearts were filled with fear and tears came to our eyes. So we said, “O Messenger of Allah! It is as though this is a farewell sermon, so counsel us.” He (peace and blessings of Allah be upon him) said, “I counsel you to have taqwa (fear) of Allah, and to listen and obey [your leader], even if a slave were to become your ameer. Verily he among you who lives long will see great controversy, so you must keep to my Sunnah and to the Sunnah of the Khulafa ar-Rashideen (the rightly guided caliphs), those who guide to the right way. Cling to it stubbornly [literally: with your molar teeth]. Beware of newly invented matters [in the religion], for verily every bidah (innovation) is misguidance.” [Abu Dawud]
On the authority of Abu Muhammad Abdullah bin ’Amr bin al-’Aas (may Allah be pleased with him) who said: The Messenger of Allah (peace and blessings of Allah be upon him) said, “None of you [truly] believes until his desires are subservient to that which I have brought.”