Quran 9.35-36
On the day when it will (all) be heated in the fire of hell, and their foreheads and their flanks and their backs will be branded therewith (and it will be said unto them): Here is that which ye hoarded for yourselves. Now taste of what ye used to hoard. Lo! the number of the months with Allah is twelve months by Allah's ordinance in the day that He created the heavens and the earth. Four of them are sacred: that is the right religion. So wrong not yourselves in them. And wage war on all of the idolaters as they are waging war on all of you. And know that Allah is with those who keep their duty (unto Him).