At-Tawba 9:37

The postponement of [Sacred Months] is an act of disbelief[35], which leads them further astray. They declare a month lawful in one year and sacred the next year, in order to match the number of months that Allah has declared as sacred, thus making lawful what Allah has made forbidden[36]; their evil deeds have been made appealing to them, and Allah does not guide the disbelieving people.

[35] Fighting during a sacred month.
[36] If they found it advantageous to violate a sacred month, they would do so, designating another month sacrosanct instead to observe the restrictions of fighting.

Indeed, the postponing [of restriction within sacred months] is an increase in disbelief by which those who have disbelieved are led [further] astray. They make it[473] lawful one year and unlawful another year to correspond to the number made unlawful by Allāh[474] and [thus] make lawful what Allāh has made unlawful. Made pleasing to them is the evil of their deeds; and Allāh does not guide the disbelieving people.

[473] Fighting during a sacred month.
[474] If they found it advantageous to violate a sacred month, they would do so, designating another month in its place in which to observe the restrictions concerning fighting.

37. The postponing (of a Sacred Month) is indeed an addition to disbelief: thereby the disbelievers are led astray, for they make it lawful one year and forbid it another year in order to adjust the number of months forbidden by Allâh, and make such forbidden ones lawful. The evil of their deeds is made fair-seeming to them. And Allâh guides not the people who disbelieve.

Postponement (of a sacred month) is only an excess of disbelief whereby those who disbelieve are misled; they allow it one year and forbid it (another) year, that they may make up the number of the months which Allah hath hallowed, so that they allow that which Allah hath forbidden. The evil of their deeds is made fairseeming unto them. Allah guideth not the disbelieving folk.