Imam al-Shafi'i (may Allah have mercy on him) said: "Allah, exalted be His praise, says: 'Then when the sacred months have passed, kill the polytheists wherever you find them.' [Surah al-Tawbah: 5]
And He said: 'And fight them until there is no more fitnah and the religion is wholly for Allah.' [Surah al-Anfal: 39]"
Sayyiduna Abu Hurayrah (may Allah be pleased with him) says 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). Whoever says 'La ilaha illallah' (There is no god but Allah) has protected his wealth and his life from me, except for the rights of Islam, and his reckoning is with Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18623