The Prophet Muhammad has left behind the Quran and the Sunna for guidance, both were equally inspired from God upon which legislation can be made.
He (Allah) it is Who sent among the unlettered ones a Messenger (Muhammad) from among themselves, reciting to them His Verses, purifying them, and teaching them the Book (the Quran) and the Wisdom (i.e. the Sunna). And verily they had been before in manifest error. (Quran 62:2)
"I have been given the Quran and its equal (the Sunna) with it." (Hadith)
The Sunna consists of the comprehensive words, actions and tacit approvals and descriptions of the Prophet Muhammad that are authentically attributed to him and must be acted upon.
Say: "If you (really) love Allah, then follow me, Allah will love you and forgive you your sins. (Quran 3:31)
"...The character of the Apostle of Allah was the Quran." (Hadith)
To rely only upon the Quran and reject, philosophically deny or ridicule the authentic Hadeeth of the Prophet is disbelief itself.
...We have explained in detail Our Revelations for people who understand. (Quran 6:98)
"Let me not find any of you, while resting on his couch, that an order of mine is mentioned in front of him, either a command of mine or a prohibition, and he says: "I do not know! Whatever we find in the Book of Allah (the Quran) we follow it (otherwise we do not)." (Hadith)
The authentic Sunna of the Prophet never contradicts but explains the Quran clearly as the Sunna was also inspired by Allah.
Nor does he speak of his own desire, it is only a Revelation revealed... (Quran 53:3-4)
Abu Dharr رضي الله عنه said: "Allah's Messenger had left us in the following state: that there was not even a bird flapping its wings in the air without his having already imparted to us knowledge about it. Then he said: 'Nothing remains that brings you closer to Paradise or further from the Hellfire except that it has been explained to you'." (Hadith)
Allah has preserved the Sunna of the Prophet Muhammad as the methodology to follow along with the Quran for all of mankind until the end times.
Verily, I revealed the Remembrance and surely I will preserve it. (Quran 15:9)
"Surely, Allah will send for this umma at the advent of every one hundred years a person (or persons) who will renovate its religion for it." (Hadith)
The only difference between the "Remembrance" (Quran and Sunna) sent by Allah is that the Quran is recited and listened to as an act of worship whereas the Sunna is not.
So when the Quran is recited, listen to it, and be silent that you may receive mercy. And remember your Lord by your tongue and within yourself, humbly and with fear... (Quran 7:204-5)
Any form of worship not found in the Quran and authentic Sunna is rejected as an innovation (bid'a).
"...The best of the speech is embodied in the Book of Allah, and the beet of the guidance is the guidance given by Muhammad. And the most evil affairs are their innovations; and every innovation is error." (Hadith)
All disputes and problems that arise are to be resolved by referring to the comprehensive Quran and Sunna. The authentic hadeeth override all contradictory derived opinions found in all the schools of Islamic jurisprudence.
O you who believe! Obey Allah and obey the Messenger, and those of (you Muslims) who are in authority. And if you differ in anything amongst yourselves, refer it to Allah and His Messenger (i.e. the Quran and the Sunna), if you believe in Allah and the Last Day. That is better and more suitable for final determination. (Quran 4.59)
Beliefs and legislation can be derived from authentic Hadeeth narrations whether transmitted by many (muttawaatir) Companions (Sahabah) of the Prophet or even by one (ahad).
"While some people were at Quba (offering) morning prayer, one man came to them and said, "Last night Quranic Verses have been revealed whereby the Prophet has been ordered to face the K'aba (at Makkah), so you too should face it too." So they, keeping their postures, turned towards the K'aba. Formerly the people were facing Shaam (Jerusalem). (Allah said): And from wheresoever you start forth (for prayers), turn your face in the direction of Sacred Mosque (of Makkah), and wheresoever you are turn your face towards it (when you pray)... (Quran 2.150)
The scholars of the science of hadith verify the authenticity of the statements attributed to the Prophet by rigorously checking the chains of narration (isnaad). (e.g. Hadith collections by scholars such as Muhammad Ismail al-Bukhari).
"Verily, Allah does not take away knowledge by snatching it from the people but He takes away knowledge by taking away the Scholars, so that when He leaves no learned person, people turn to ignorant as their, leaders; then they are asked to deliver religious verdicts and they deliver them without knowledge, they go astray, and lead others astray." (Hadith)