The Quran and Sunnah can not be separated.
Allah gave complete knowledge of the religion to the Prophet ﷺ as revelation along with the Quran. This is why companions would flock to the Prophet ﷺ and ask him for the meaning of verses rather than just saying everyone’s opinion and interpretation of the Quran is equal and that’s it. Even looking at it by secular standards there is undeniable and massive amounts of evidence which has been mass reported of the Companions and their students doing the same, relying on the Sunnah together with the Quran.
Allah mentions the Quran and Sunnah together and then refers to people like this who would separate between them and those similar to them in the same verse, when He says: ❝And if it was not for the favor of Allah upon you and His mercy, a group of them would have determined to mislead you. But they do not mislead except themselves, and they will not harm you at all. And Allah has revealed to you the Book (Quran) and al-Hikmah (Sunnah) and has taught you that which you did not know. And ever has the favor of Allah upon you been great.❞ [Surat an-Nisa’, 4:113]
The Prophet ﷺ specifically mentioned this would happen in later generations after him when he said, ❝Indeed! Soon a hadith of mine will be related to a man, while he is reclining on his couch, who will say, ‘The Book of Allah (Quran) is enough between you and us. Whatever we find in it as halal we accept as halal, and whatever we find in it as haram we accept as haram.’ Behold, whatever Allah’s Messenger has declared as haram is just like that which Allah has declared haram.❞ [Tirmidhi 41/20]