Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) says: Sayyiduna Ali (may Allah be pleased with him) sent me to the people of al-Nahr to fight the Khawarij, and I invited them (to return) three times before we fought them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16739