Nahjul Balagha Part 2, Letters and Sayings
Letter 77: Instructions to Abdullah b. Abbas
Instructions given to `Abdullah ibn al-`Abbas, at the time of his being deputed to confront the Kharijites
ومن وصيته (عليه السلام) لَهُ
لما بعثه للا حتجاج على الخوارج
Do not argue with them by the Qur'an because the Qur'an has many faces. You would say your own and they would say their own; but argue with them by the sunnah, because they cannot find escape from it.
لاَ تُخَاصِمْهُمْ بِالْقُرْآنِ، فَإِنَّ الْقُرْآنَ حَمَّالٌ ذُو وُجُوه، تَقُولُ وَيَقُولُونَ، وَلكِنْ حاجّهُمْ بالسُّنَّةِ، فَإِنَّهُمْ لَنْ يَجِدُوا عَنْهَا مَحِيصاً.