Nafi' relates that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) used to greet the Khashabiyyah and the Khawarij with salam while they were fighting each other. He would say: "Whoever says, 'Come to prayer,' I respond to him; and whoever says, 'Hayya 'ala al-falah' (Come to success), I respond to him as well; but whoever says, 'Come to killing your Muslim brother and seizing his wealth,' I will refuse."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5305