Surah At-Tawbah — Ayah 12 (9:12)
وَإِن نَّكَثُوٓاْ أَيۡمَٰنَهُم مِّنۢ بَعۡدِ عَهۡدِهِمۡ وَطَعَنُواْ فِي دِينِكُمۡ فَقَٰتِلُوٓاْ أَئِمَّةَ ٱلۡكُفۡرِ إِنَّهُمۡ لَآ أَيۡمَٰنَ لَهُمۡ لَعَلَّهُمۡ يَنتَهُونَ ١٢
English translation (Saheeh International)
And if they break their oaths after their treaty and defame your religion, then combat the leaders of disbelief, for indeed, there are no oaths [sacred] to them; [fight them that] they might cease.
Brief commentary
If these idolaters, with whom you entered into a treaty not to fight for a set period, break their oaths and promises and insult your religion and attack it, then fight them, because they are the leaders of disbelief and there are no treaties or promises that can protect their lives. Fight them in the hope that they will stop their disbelief, violation of their treaties and speaking badly of the religion.