مـسـنـد الإمـام الـربـيـع بن حـبـيـب و كـتـاب الـجـامـع لإبن بـركـة، دراسـة مـقـارنـة
Note: Those who aren't well versed and grounded with the `Aqeedah and basics of Islam should not go to their websites lest they deviate or shaytaan infiltrate their heart with doubts. The Khawarij are a deviant sect and all of it's descendents are dangerous for a Muslim. You can see from their speech here how different they are from the mainstream Muslims even in the way they speak and the way they quote from whom they quote : https://www.youtube.com/watch?v=iAOkrQCT3Jc. He reviled and mocked `Amr ibn al-`Aas in this video at 2:28; May allah guide them
The `Ibadhis describe this book as follows :