Abdullah ibn Abbaas said, "Do not sit with the people of innovation, for verily their sittings are a sickness for the hearts." [ash-Shari'ah pg 65 of al-Aajurree (d.360)]
"A man said to Abdullah ibn Umar: "Najdah (a man from the Khawaarij) says such and such". So Ibn Umar prevented himself from listening for fear that some of it should enter his hearts". [Related by al-Laalikaaee no.199]
Renouncing the innovators and boycotting their innovation
In safeguarding the interests of damages of the innovators, Islam approved and legalized the renouncement and boycotting of the innovators. The Salaf, (ra) adopted this method in the fight against innovation and renounced the innovators in order to safeguard the Shari'ah. However, in order not to fight an innovation with another, their renouncement was based on faith and adherence to the Qur'an and the Sunnah.
"Renouncement should not be adopted unless there is reasonable grounds for it and this entails the following:
a. To be sure of the existence of innovation and avoid hearsay. This is to say that one should listen to what the innovator says or does, or read what he has written, particularly in our era which lacks piety.
b. To fight the type of innovation which is agreed upon. This is to say that matters on which the scholars have different views whether they are innovations or not should not be renounced.
c. To clarify the issue to the person who says or practices an innovation before calling him an innovator. This person may be excused because he was not aware of the evidence he is opposing, or may have understood it mistakenly. This clarification can only be achieved through advising him and calling him to the Qur'an and the Sunnah with kindness." [Haqeeqatu Al-Bida'ah Wa Ahkaamuhaa (The Reality of Innovation and Its Principles, 2/339)]
source: http://safia71.tripod.com/bida'_ah.htm (in depth)
