Musannaf Ibn Abi Shayba 38:146

Yahya ibn Adam narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Ma’mar, from Rib’i, from Tawus, from his father, from Ibn ‘Abbas, that he mentioned what the Khawarij encounter regarding the Qur’an. He said: "They believe in its clear verses (muhkam), and they perish at its ambiguous verses (mutashabih)."