The Cow - 2:194
The holy month can only be observed on both sides. The observance of the truce is mutual. If they attack you, attack them in kind. Listen to Allah and know that Allah is with the righteous.
How many holy months are there?
The Repentance/9:36
The number of months in Allah´s Book on the day He created the heavens and the earth is twelve according to Allah. Four of them are sacred. This is the perfect religion; do not wrong yourselves in those months (by fighting). But if the idolaters fight you in a total war, then fight them in a total war. Know that Allah is with the righteous.