The All-Knowing, All-Wise Lord did not send any of His Books alone.but rather He, in His Absolute Wisdom and Knowledge always appointed a Prophet amongst the people and revealed His Divine Guidance upon Him so that His Messengers would not only convey the Message of their Lord, but live and present a practical example of how the Message of the Lord is to be implemented in their lives and society.
Thus if one reads the Glorious Quran with understanding one will and can very well understand the basic concepts of faith like the Absolute Oneness of Allah Subhanah, the concept of the Hereafter, the Paradise, the Hell Fire, etc..but to implement the Shariah or Laws of Islam upon their social and individual lives, it is absolutely imperative that one look into, learn and study and strive to implement the Sunnah or the practice of the Messenger of Allah (saws) into their lives.
Thus if any amongst mankind states the Quran alone is enough for him without the understanding and implementation of the Sunnah of the Messenger of Allah (saws) into their lives, his knowledge as well as his practice of Islam would be absolutely incomplete, to say the least.
Allah says in the Holy Quran Chapter 4 Surah Nisa verse 79-80:
We have sent you (O Mohamed) as a Messenger to mankind, and Allahs witness suffices for this. Whoever obeys the Messenger, in fact obeys Allah, and whoever turns away from him, he will be accountable for his deeds. Anyhow, we have not sent you (O Mohamed) to be a guardian over them.
Allah says in the Holy Quran Chapter 3 Surah Ale Imran verse 31-32:
O Prophet, tell the people, If you sincerely love Allah, follow me. Then Allah will love you and forgive you your sins, for He is All Forgiving, All Merciful. Also say to them, Obey Allah and His Messenger. And if, in spite of this, they do not accept your invitation, warn them that Allah does not like the disbelievers..
Thus without an iota of a doubt, to accept, obey, follow and implement the authentic and established Sunnah of the Messenger of Allah (saws) is an integral and inseparable part of being a muslim.
If one trusts, obeys, and follows the guidance and commands of Allah and His Messenger (saws), one can be assured of never ever being misled; but if one believes, obeys and follows any other guidance, other than that of Allah and His Messenger (saws), one can be assured of being led astray.
Whatever written of Truth and benefit is only due to Allahs Assistance and Guidance, and whatever of error is of me alone. Allah Alone Knows Best and He is the Only Source of Strength.