Indeed, the number of months with Allāh is twelve [lunar] months in the register of Allāh [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion [i.e., way], so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allāh is with the righteous [who fear Him].

Sahih International