The only hadith the followers of the Quran are to follow is the Quran. To follow any other hadith besides the Quran is to go against the very verses of the Quran itself. For this, God provides us an ultimatum, either we follow the Quran as our only Hadith or we follow some other sources.
[45:6] These are GOD’s revelations that we recite to you truthfully. In which Hadith other than GOD and His revelations do they believe?
تِلْكَ آيَاتُ اللَّهِ نَتْلُوهَا عَلَيْكَ بِالْحَقِّ فَبِأَيِّ حَدِيثٍ بَعْدَ اللَّهِ وَآيَاتِهِ يُؤْمِنُونَ
[7:185] Have they not looked at the dominion of the heavens and the earth, and all the things GOD has created? Does it ever occur to them that the end of their life may be near? Which Hadith, beside this, do they believe in?
أَوَلَمْ يَنْظُرُوا فِي مَلَكُوتِ السَّمَاوَاتِ وَالْأَرْضِ وَمَا خَلَقَ اللَّهُ مِنْ شَيْءٍ وَأَنْ عَسَىٰ أَنْ يَكُونَ قَدِ اقْتَرَبَ أَجَلُهُمْ فَبِأَيِّ حَدِيثٍ بَعْدَهُ يُؤْمِنُونَ
[77:50] Which Hadith, other than this, do they uphold?
فَبِأَيِّ حَدِيثٍ بَعْدَهُ يُؤْمِنُونَ
God equates such people who choose to follow additional sources beyond what God instituted in the scripture as idol worshipers, because the only religious laws we are to follow are those that came from God. If someone accepts religious laws from another source besides God, then they are confirming that they have set up another god beside God.
[6:121] Do not eat from that upon which the name of GOD has not been mentioned, for it is an abomination. The devils inspire their allies to argue with you; if you obey them, you will be idol worshipers.
وَلَا تَأْكُلُوا مِمَّا لَمْ يُذْكَرِ اسْمُ اللَّهِ عَلَيْهِ وَإِنَّهُ لَفِسْقٌ وَإِنَّ الشَّيَاطِينَ لَيُوحُونَ إِلَىٰ أَوْلِيَائِهِمْ لِيُجَادِلُوكُمْ وَإِنْ أَطَعْتُمُوهُمْ إِنَّكُمْ لَمُشْرِكُونَ
Hadith are apparent testimonies collected from individuals who lived some 200 years after the prophet that they claim trace back to the prophet. If we accept another testimony over what God informs us in the Quran, then this equates to idol worship. This is confirmed in the following verse.
[6:19] Say, “Whose testimony is the greatest?” Say, “GOD’s. He is the witness between me and you that this Quran has been inspired to me, to preach it to you and whomever it reaches. Indeed, you bear witness that there are other gods beside GOD.” Say, “I do not testify as you do; there is only one god, and I disown your idolatry.”
قُلْ أَيُّ شَيْءٍ أَكْبَرُ شَهَادَةً قُلِ اللَّهُ شَهِيدٌ بَيْنِي وَبَيْنَكُمْ وَأُوحِيَ إِلَيَّ هَٰذَا الْقُرْآنُ لِأُنْذِرَكُمْ بِهِ وَمَنْ بَلَغَ أَئِنَّكُمْ لَتَشْهَدُونَ أَنَّ مَعَ اللَّهِ آلِهَةً أُخْرَىٰ قُلْ لَا أَشْهَدُ قُلْ إِنَّمَا هُوَ إِلَٰهٌ وَاحِدٌ وَإِنَّنِي بَرِيءٌ مِمَّا تُشْرِكُونَ
Additionally, this is a sign that such individuals who follow other sources besides God’s scripture do not believe in the Hereafter.
[6:112] We have permitted the enemies of every prophet—human and jinn devils—to inspire in each other fancy words, in order to deceive. Had your Lord willed, they would not have done it. You shall disregard them and their fabrications. [6:113] This is to let the minds of those who do not believe in the Hereafter listen to such fabrications, and accept them, and thus expose their real convictions.
This is because people who follow additional sources beyond God’s scripture prove that they do not believe in God’s words in the scripture, such that the Quran is complete, fully detailed, and the only source of religious laws decreed for the believers. This is in addition that we are commanded not to follow any other hadith beside the Quran, as mentioned above.
[6:114] Shall I seek other than GOD as a source of law, when He has revealed to you this book fully detailed? Those who received the scripture recognize that it has been revealed from your Lord, truthfully. You shall not harbor any doubt.
أَفَغَيْرَ اللَّهِ أَبْتَغِي حَكَمًا وَهُوَ الَّذِي أَنْزَلَ إِلَيْكُمُ الْكِتَابَ مُفَصَّلًا وَالَّذِينَ آتَيْنَاهُمُ الْكِتَابَ يَعْلَمُونَ أَنَّهُ مُنَزَّلٌ مِنْ رَبِّكَ بِالْحَقِّ فَلَا تَكُونَنَّ مِنَ الْمُمْتَرِينَ
[6:115] The word of your Lord is complete, in truth and justice. Nothing shall abrogate His words. He is the Hearer, the Omniscient.
وَتَمَّتْ كَلِمَتُ رَبِّكَ صِدْقًا وَعَدْلًا لَا مُبَدِّلَ لِكَلِمَاتِهِ وَهُوَ السَّمِيعُ الْعَلِيمُ