Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "My ummah will split into two groups, and from between them a group will break away (from the religion); it will be killed by whichever of the two groups is closer to the truth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16779