At-Tawbah 9:36 ان عدة الشهور عند الله اثنا عشر شهرا في كتاب الله يوم خلق السماوات والارض منها اربعة حرم ذالك الدين القيم فلا تظلموا فيهن انفسكم وقاتلوا المشركين كافة كما يقاتلونكم كافة واعلموا ان الله مع المتقين ٣٦

Verily, the number of months with Allâh is twelve months (in a year), so was it ordained by Allâh on the Day when He created the heavens and the earth; of them four are Sacred (i.e. the 1st, the 7th, the 11th and the 12th months of the Islâmic calendar). That is the right religion, so wrong not yourselves therein, and fight against the Mushrikûn (polytheists, pagans, idolaters, disbelievers in the Oneness of Allâh) collectively¹ as they fight against you collectively. But know that Allâh is with those who are Al-Muttaqûn (the pious - See V.2:2).