Why do we have to follow the sunnah of Prophet Muhammad and not just follow the Qur'an?
The question may appear strange and somewhat surprising to committed, practising Muslims. How can something which is so obviously one of the bases of Islaam become a matter for discussion and debate? But since the question has been asked, we will present, with the help of Allah, the principles and bases of the importance of the Sunnah, the obligation to follow it and the ruling concerning those who reject it. By so doing, we will also refute the doubters and the misguided group who call themselves “Submitters” (the Qur′an has nothing to do with them!) Insha'Allah this discussion will be of benefit to everyone who wants to understand the truth of the matter.
Proof of the importance of the Sunnah from the Qur'an itself:
Allah says (interpretation of the meaning): “He who obeys the Messenger has indeed obeyed Allah...” [4:80] Allah described obedience to the Prophet (peace be upon him) as being a part of obedience to Him. Then He made a connection between obedience to Him and obedience to the Prophet (peace be upon him): “O you who believe! Obey Allah and obey the Messenger...” [4:59]
Allah warns us not to go against the Prophet (peace be upon him), and states that whoever disobeys him will be doomed to eternal Hell. Allah says (interpretation of the meaning): “...And let those who oppose the Messenger′s commandment beware, lest some fitnah (trial, affliction, etc.) befall them or a painful torment be inflicted on them.” [24:63]
Allah has made obedience to His Prophet a religious duty; resisting or opposing it is a sign of hypocrisy:
”But no, by your Lord, they can have no Faith, until they make you [Muhammad] judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission.” [4:65]
Allah commands His slaves to respond to Him and His Messenger:
“O you who believe! Answer Allah (by obeying Him) and (His) Messenger when he calls you to that which will give you life...” [8:24]
Allah also commands His slaves to refer all disputes to him:
“... (And) if you differ in anything amongst yourselves, refer it to Allah and His Messenger...” [4:59]
Allah also calls all people who accept Allah but refute the Messenger as 'Real Kafirs': “Verily, those who deny Allah and His Messengers and (those who) wish to separate Allah from His Messengers (by believing in Allah and disbelieving in His Messengers), saying: "We believe in some but reject others": and (those who) wish to adopt a way in between. They are in truth (equally) Unbelievers; and We have prepared for Unbelievers a humiliating Punishment. To those who believe in Allah and His Messengers and make no distinction between any of the Messengers, We shall soon give their (due) rewards: for Allah is Oft-Forgiving Most Merciful.” [4:150-153]
In a nutsell: Islam = Qur'an + Sunnah only. You will see that all groups who deviated from Islam are violating this rule. Shi'a replace the Sunnah with their Imams and some fabricated narrations. Extremist Sufis replace the Qur'an and the Sunnah with their Shaikhs (leaders). The Submitters reject the Sunnah from the equation and claim to follow the Qur'an only. All of those who reject the Sunnah are NOT Muslims.