Abū ‘Uthmān Sa’īd b. Ismā’īl Al-Naysābūrī (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 tabāraka wa ta’ālā said:

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

Abū Nu’aym in Al-Ḥilyah Vol.10 p244, and others.

After mentioning this and other similar narrations Shaykh Al-Islām Ibn Taymīyah said (Minhāj 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.