According to your Question you regard "that the Holy Quran is the single most important source of information" as you said.
I have a "sensibility" about the word "Holy" but i regard that Quran is our most and 1st important source. And i must admit i have also a kind of sensibility about some ahadith (hadiths) that even might be regarded as sahih but i will never reject all of them! And as the rules of sahih ahadith are very clear and hard i must accept that sahih hadiths are the 2nd important source ... One should always be careful and check if a Hadith isn't contradictory to a Quranic Verse!
To start we should know that Allah said in (16:44)
And We revealed to you the message that you may make clear to the people what was sent down to them and that they might give thought.
So this means that Quran can't be understood as a standalone source, we need to know how the Prophet (Peace be upon him) practiced it's teaching, therefore some madhabs had a ranking of Sunna in their osol, where the practiced Sunna (ahadith where it is explained what and how the Messenger of Allah (Peace be upon him) used to practice something) is much higher then the verbal Sunna (hadith with verbal orders or prohibitions)!
This Verse is as i think very clear about the matter that Sunna (Hadith) is an explanation (often practical) in how the Quran should be understood and as you read the Prophet (peace be upon him)n is asked to explain and teach it to them. Explaining is what we find in the Sunna!
I will now quote some more verses of the Quran where the Sunna is indicated and try to refer to them later!
- Lord, raise up from among them a Messenger who shall recite Your Revelations to them and teach them the Book الْكِتَابَ and Wisdom الْحِكْمَةَ and purify their lives. You art All-Powerful and All-Wise"
[Surat al-Baqara 2:129]
- just as (you have found from this: that) We sent the Messenger to you from among you, who recites to you Our Revelations آيَاتِنَا; who purifies your lives; who instructs you in the Book الْكِتَابَ and in Wisdom الْحِكْمَةَ and teaches you those things that you did not know.
[Surat al-Baqara 2:151]
- As a matter of fact, it is the great favour of Allah to the Believers that He has raised up in their midst from among themselves a Messenger who recites to them His Revelations آيَاتِهِ, purifies their lives and teaches them the Book الْكِتَابَ and Wisdom الْحِكْمَةَ, though before this these very people had been in manifest error.
[Surat Al-I'mran 3:164]
- ... Allah has sent down the Book and the Wisdom ...
[An-Nissa' 4:113]
وَأَنزَلَ اللّهُ عَلَيْكَ الْكِتَابَ وَالْحِكْمَةَ
- Tell them plainly, "This is my way. I invite you to Allah. I myself see my Way in full light and my followers see it, too. And Allah is free from any defect, and I have nothing to do with those who practice shirk."
[Surat Yusuf 12:108]
- And remember what is recited in your houses of the verses of Allah آيَاتِ اللَّهِ and wisdom الْحِكْمَةِ.
[Surat al-Ahzaab 33:34]
- He it is Who has raised a Messenger among the gentiles from among themselves, who recites to them His Revelations آيَاتِهِ, purifies them and teaches them the Book الْكِتَابَ and the Wisdom الْحِكْمَةَ, whereas before this they were indeed lost in manifest error.
[Surat al Jumu'a 62:2]
In most of these verses (1, 2, 3, 4, 6 and 7) you may find the words the book الْكِتَاب and Wisdom الْحِكْمَة: In most tafassir the book is interpreted as the Quran and the Wisdom is interpreted as the sunna (according to Qatada) or as the knowledge about the Ahkam of islam (halal/haram etc.). Now as the messenger of Allah (peace be upon him) is the one who teaches us this Wisdom it's clear that Wisdom and Sunna overlap, as without what our Prophet taught us we wouldn't know the number of raka'at in the Prayer at what time to pray and manasik al Hajj and many many things.
The 5th Verse in the above list is only an example of that one should follow the Messenger of Allah (peace be upon him) (there might be more expressive Verses for example 59:7) and following him means doing as he did this means in Arabic following his Sunna السنة!
The 6th Verse is also very expressive as it says that the Women of the Prophet (peace be upon him) should spread out the Wisdom and the revelations of Allah -which they heared/saw in their houses- to people!
All these verses (and many more) indicate the necessity of Hadith and Sunna.
At least here is a Hadith narrated in other Versions in other books mentioning that Quran isn't the only source of islamic wisdom!
Narrated Al-Miqdam ibn Ma'dikarib: The Prophet (ﷺ) said: Beware! I have been given the Qur'an and something like it, yet the time is coming when a man replete on his couch will say: Keep to the Qur'an; what you find in it to be permissible treat as permissible, and what you find in it to be prohibited treat as prohibited. Beware! The domestic ass, beasts of prey with fangs, a find belonging to confederate, unless its owner does not want it, are not permissible to you If anyone comes to some people, they must entertain him, but if they do not, he has a right to mulct them to an amount equivalent to his entertainment.
حَدَّثَنَا عَبْدُ الْوَهَّابِ بْنُ نَجْدَةَ، حَدَّثَنَا أَبُو عَمْرِو بْنُ كَثِيرِ بْنِ دِينَارٍ، عَنْ حَرِيزِ بْنِ عُثْمَانَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي عَوْفٍ، عَنِ الْمِقْدَامِ بْنِ مَعْدِيكَرِبَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ قَالَ " أَلاَ إِنِّي أُوتِيتُ الْكِتَابَ وَمِثْلَهُ مَعَهُ أَلاَ يُوشِكُ رَجُلٌ شَبْعَانُ عَلَى أَرِيكَتِهِ يَقُولُ عَلَيْكُمْ بِهَذَا الْقُرْآنِ فَمَا وَجَدْتُمْ فِيهِ مِنْ حَلاَلٍ فَأَحِلُّوهُ وَمَا وَجَدْتُمْ فِيهِ مِنْ حَرَامٍ فَحَرِّمُوهُ أَلاَ لاَ يَحِلُّ لَكُمْ لَحْمُ الْحِمَارِ الأَهْلِيِّ وَلاَ كُلُّ ذِي نَابٍ مِنَ السَّبُعِ وَلاَ لُقَطَةُ مُعَاهِدٍ إِلاَّ أَنْ يَسْتَغْنِيَ عَنْهَا صَاحِبُهَا وَمَنْ نَزَلَ بِقَوْمٍ فَعَلَيْهِمْ أَنْ يَقْرُوهُ فَإِنْ لَمْ يَقْرُوهُ فَلَهُ أَنْ يُعْقِبَهُمْ بِمِثْلِ قِرَاهُ " .
*In Arabic: it said I have been given the book so the translator used the interpretation kitab=book=Quran!