According to the Holy Quran, Muslims are to follow only this Holy Book.
According to the Holy Quran, Muslims are to follow only this Holy Book. The following verses from the Holy Quran are relevant:
39.23. Allah has sent down the best statement, a Book (this Qur'an), its parts resembling each other in goodness and truth, oft-repeated. The skins of those who fear their Lord shiver from it (when they recite it or hear it). Then their skin and their heart soften to the remembrance of Allah. That is the guidance of Allah. He Guides therewith whom He pleases and whomever Allah sends astray, for him there is no guide.
Is he then, who will confront with his face the awful torment on the Day of Resurrection (as he who enters peacefully in Paradise)? And it will be said to the Zalimun
(polytheists and wrong-doers, etc.): "Taste what you used to earn!"
Those before them belied, and so the torment came on them from directions they perceived not.
So Allah made them to taste the disgrace in the present life, but greater is the torment of the Hereafter if they only knew!
And indeed We have put forth for men, in this Qur'an every kind of similitude in order that they may remember.
An Arabic Qur'an, without any crookedness (therein) in order that they may avoid all evil which Allah has ordered them to avoid, fear Him and keep their duty to Him.
45.6. These are the Ayat (proofs, evidences, verses, lessons, revelations, etc.) of Allah, which We recite to you (O Muhammad ) with truth. Then in which speech after Allah and His Ayat will they believe?
45.7. Woe to every sinful liar,
45.8. Who hears the Verses of Allah (being) recited to him, yet persists with pride as if he heard them not. So announce to him a painful torment!
45.9. And when he learns something of Our Verses (this Qur'an), he makes them a jest. For such there will be a humiliating torment.
45.10. In front of them there is Hell, and that which they have earned will be of no profit to them, nor (will be of any profit to them) those whom they have taken as Auliya' (protectors, helpers, etc.) besides Allah. And theirs will be a great torment.
45.11. This (Qur'an) is a guidance. And those who disbelieve in the Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) of their Lord, for them there is a painful torment of Rijz (a severe kind of punishment).
45.12. Allah it is He Who has subjected to you the sea, that ships may sail through it by His Command, and that you may seek of His Bounty, and that you may be thankful,
45.13. And has subjected to you all that is in the heavens and all that is in the earth; it is all as a favour and kindness from Him. Verily, in it are signs for a people who think deeply.
45.14. Say (O Muhammad) to the believers to forgive those who (harm them and) hope not for the Days of Allah (i.e. His Recompense), that He may recompense people according to what they have earned (i.e. to punish these disbelievers, who harm the believers).
45.15. Whosoever does a good deed, it is for his ownself, and whosoever does evil, it is against (his ownself). Then to your Lord you will be made to return.
45.16. And indeed We gave the Children of Israel the Scripture, and the understanding of the Scripture and its laws, and the Prophethood; and provided them with good things, and preferred them above the 'Alamin (mankind and jinns) (of their time, during that period),
45.17. And gave them clear proofs in matters [by revealing to them the Taurat (Torah)]. And they differed not until after the knowledge came to them, through envy among themselves. Verily, Your Lord will judge between them on the Day of Resurrection about that wherein they used to differ.
45.18. Then We have put you (O Muhammad) on a plain way of (Our) commandment [like the one which We commanded Our Messengers before you (i.e. legal ways and laws of the Islamic Monotheism)]. So follow you that (Islamic Monotheism and its laws), and follow not the desires of those who know not. [Tafsir At-Tabari Vol. 25, Page 146].