Imaam Ibnul Jawzi (rahimahullaah) said:

The khawaarij have not ceased rebelling against the (Muslim) rulers and they have different Madhabs. The followers of Naafi Ibn Al-Azraq used to say (about themselves): ‘’We are polytheists as long as we are living in the land of disbelief (i.e. in a land of the disbelievers) and when we leave, then we are Muslims.’’  They said: ‘’those who oppose our Madhab are polytheists; those who commit major sins (besides shirk) are polytheists; those who stay behind and do not agree with us in fighting against (the Muslim Rulers) are people of disbelief.’’  These ones (i.e. the followers of Naafi) made lawful the killing of the women and children of the Muslims and passed the judgement of polytheism against them.

[Source: Talbees Iblees page: 104 with the checking of Al-Allaamah Zayd Bin Haadi (may Allaah preserve him)]