{ They ask you about the sacred months – is there fighting during them? Say: Fighting during them is a great sin, although hindering from the cause of Allah, disbelief in Him and the Sacred Mosque, and driving its people out of it, are even greater in the sight of Allah; and sedition is worse than killing. } Al-baqara 217

Related from Mujahid, who said: The prohibition against fighting in a sacred month is abrogated; the word of the Most High abrogated it: { And kill the idolaters wherever you find them … } and the rest of the verse [Al-tawba 5]. This is regarding the word of the Most High and Exalted: { They ask you about the sacred months – is there fighting during them? Say: Fighting during them is a great sin } and the rest of the verse. Abu Hanifa expressed this, and Abu Yusuf expressed it.