What is the ruling on those who do not follow the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Allaah obliged all Muslims to follow the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam, and to implement all that was revealed.  Allaah Says (what means):

Moreover the Prophet, sallallaahu ‘alayhi wa sallam, swore that if Moosa (Moses ) had been alive, he would have followed him. [Ahmad and others]

In another Hadeeth the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, commanded us to hold fast to his Sunnah and the way of the rightly-guided Caliphs. [At-Tirmithi]

Consequently, it is prohibited to adopt the opinion of any person who deviates from the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam. All Islamic scholars instructed us not to follow any of their opinions if proven to be contradicting the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam. 

Allaah Knows best.