wa alaykumus salaam wa rahmatullahi wa barakatuh,

The 'Ulama have stated that by disbelieving in the ahadeeth a person leaves the fold of Islam. Allamah as-Suyuti (May Allah Ta'ala have mercy on him) said, “Know -may Allah bless you- whoever denies the narrations of the Prophet صلي الله عليه وسلم either in word or deed (with the conditions that they are known) as a hujjah (proof), then he is an infidel, out of the Islamic faith and will be raised with Jews and Christians or people who Allah wants from the infidels.” (Miftah al-Jannah, 8) For more details, see http://www.muftisays.com/forums/the-true-salaf-as-saliheen/6652/-ruling-on-those-who-reject-ahadith.html

Mufti Ebrahim Desai said, ' To reject a Hadith Mutawaatir is Kufr. To reject Hadith Aahaad (which has many branches in relation to the standard of its authenticity) is Fisq (transgression).' see http://www.islam.tc/cgi-bin/askimam/ask.pl?q=4883&act=view

Shaykh Abdul Azeez Ibn Baaz (rahimahullah) said, 'The denial of the sunnah is an act of kufr and apostasy.' [In defense of the Qur'an and Sunnah, p5]

Ibn Hazm (rahimahullah) said, "If a person says, 'We only follow what we find in the Qur'an,' then he has become a disbeliever according to the consensus of this nation [of Islam]." [Quoted in Maalim as-Sunnah an-Nabawiyya]

Ibn Battah (rahimahullah) stated, 'If a person believes in everything that the Messenger (peace be upon him) brought except for one thing, and he rejects that thing, then he is a disbeliever according to all of the scholars.' [The authority and importance of the Sunnah]

To deny the place of the sunnah in islam, to deny that it is obligatory to believe in what the Prophet (peace be upon him) said, to deny that it is obligatory to obey his commans and prohibitions or to deny that he is the example that every Muslim must follow are all acts of kufr (disbelief). If a person willingly and knowingly continues to hold such beliefs after the clear proofs have been shown and explained to him, he falls outside the fold of Islam and is a disbeliever. In this world, he is to be treated as a disbeliever. He is not deserving of the love and respect that one must give to one's brother in faith. Unless he repents before he dies, all of his deeds will be of no avail to him and in the Hereafter he will be in the Hell-fire forever. [The authority and importance of the sunnah]

Also take a look at:

The authority of hadith and sunnah in light of the holy Qur'an - http://www.croydonmosque.com/pdf/The%20Authority%20of%20Sunnah%20and%20Hadith%20in%20the%20light%20of%20the%20Holy%20Quran.pdf

The authority of sunnah (Mufti Taqi Usmani) - http://www.central-mosque.com/fiqh/asunnah1.htm

The authority and importance of the sunnah - http://www.kalamullah.com/importance-of-the-sunnah.html

was salaam