Is it obligatory to follow the Sunnah or only the Qur'an, and is a Muslim bound to follow a specific madhhab?

The Sunnah is a proof that must be followed, and this has been indicated by the Holy Quran, the Prophetic Sunnah, scholarly consensus, and sound reasoning. From the Quran, Allah commanded obedience to the Messenger and warned against disobeying him, making his obedience a prerequisite for faith. He also enjoined responding to Allah and His Messenger and referring to them in case of dispute. From the Sunnah, the Prophet (peace be upon him) warned against those who would recline and reject anything not found in the Quran, and he commanded following his Sunnah and the Sunnah of the Rightly Guided Caliphs. The Companions, the Tabi'un, and those who came after them, have unanimously agreed on accepting the Prophet's reports and establishing them as Sunnah. From a rational perspective, the fact that the Prophet is the Messenger of Allah necessitates believing him and obeying him, and it is not possible to differentiate between what he reported in the Quran and what he reported outside of it. Whoever denies the authority of the Sunnah disbelieves. As for following a specific madhhab (school of thought), this is not obligatory. Rather, a layperson must ask someone he trusts, and a student of knowledge must follow the preponderant opinion with its evidence. It is permissible to follow one of the four madhhabs, provided that one is willing to deviate from the madhhab and follow the truth if it becomes clear that it contradicts it, because the madhhabs are paths to understanding the rulings of the Book and the Sunnah, not an end in themselves.