It is narrated from Sayyiduna Jabir and Sayyiduna Abu Hurayrah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight against the people until they say 'La ilaha illallah' (There is no god but Allah). When they say it, they have protected their wealth and their lives from me, except for the rights of Islam, and their reckoning is with Allah Almighty."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18624