The most important source of legislation after the Book of Allah (سبحانه وتعالى) is the Sunnah of His Messenger (صلى الله عليه وسلم). Indeed, the shahadatayn consists of two statements; 'I testify that there is no diety worthy of worship except Allah and I testify that Muhammad is His Slave and Messenger', without these two assertions one cannnot enter through the door of Islam.
However, something so clear is becoming muddy today, we have those who disbelieve in the latter statement yet they consider themselves within the fold of Islam. Matters of belief that the whole Ummah had agreed upon are becoming debated today.
The reality is that those who disbelieve in the second statement automatically disbelieve in the first, for Allah (سبحانه وتعالى) says:
“But no, by your Lord, they can have no Faith, until they make you (O Muhammad (صلى الله عليه وسلم)) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission”
And Allah (سبحانه وتعالى) Himself defends the decisions of His Prophet,
“Nor does he speak of (his own) desire.
It is but a revelation revealed”
Therefore when one hears an authentic statement attributed to the Prophet (صلى الله عليه وسلم), it is incumbent upon him to believe in it, to willingly accept it and to act according to it, if he truly believes in that which was sent to Muhammad (صلى الله عليه وسلم).
As-Suyooti (رحمه الله) said:
You should understand, may Allah have mercy on you, that whoever denies that the hadith of the Prophet (صلى الله عليه وسلم) constitutes shar‘i evidence – whether he denies a report that speaks of something that the Prophet (صلى الله عليه وسلم) said or did, if that hadith fulfils the conditions stipulated in usool al-hadith – has committed an act of disbelief that puts him beyond the bounds of Islam, and he will be gathered (on the Day of Resurrection) with the Jews and Christians, or with whomever Allah wills of the disbelieving groups.
Miftaah al-Jannah fi’l-Ihtijaaj bi’s-Sunnah (p 14)