Imām As-Sa’di (rahimahullāh) explained the following part of āyah 73 from sūrah Āli Imrān:
قُلْ إِنَّ الْهُدَىٰ هُدَى اللَّهِ
“Verily! Right guidance is the Guidance of Allah” (Āli- Imrān: 73)
This verse and those similar to it, show us that whomsoever seeks guidance and the correct way from other than the Qur’ān and the Sunnah, then he has become misguided and left the correct path. Because (true) guidance is restricted to the guidance of Allāh, that which he sent with the Messenger (sallallāhu alaihi wa salam).
Fath ar-Rahīm al-Malik al-‘Alām fī ‘ilm al-‘Aqā’id wa at-Tawhīd wa al-Akhlāq wa al-Ahkhām al-Mustambita min al-Qur’ān – pg 216