Holding to the Prophet's way and leaving innovation in the religion.
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 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.”
About this text. Arabic and English text from the hadith-api project (github.com/fawazahmed0/hadith-api), editions ara-nawawi and eng-nawawi, released into the public domain under the Unlicense. References and gradings are Imam an-Nawawi's own, as they appear in the text.