Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "I have been commanded to fight people until they bear witness to the testimony of faith. When they say it, they have protected their blood and their wealth from me, except by its right, and their reckoning is with Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16827