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]"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / Q18623