Abû ‘Uthmân Sa’îd b. Ismâ’îl Al-Naysâbûrî (d290H) said:

Whoever governs himself by the Sunnah – in word and deed – will speak with wisdom. But whoever governs himself according to his own desires will speak heresies (bid’ah); because Allah tabâraka wa ta’âlâ said:

And if you obey him (the Messenger) you will be guided. [Sûrah Al-Nûr: 54]

Abû Nu’aym in Al-Hilyah Vol.10 p244, and others.

After mentioning this and other similar narrations Shaykh Al-Islâm Ibn Taymîyah said (Minhâj Al-Sunnah Vol.5 p117):

It is as they said, for if a person does not follow what the Messenger came with he will act according to his own wishes. Thus he will be one who follows his desires without guidance from Allah.