Qur'an 9:37

إِنَّمَا ٱلنَّسِىٓءُ زِيَادَةٌ فِى ٱلْكُفْرِ ۖ يُضَلُّ بِهِ ٱلَّذِينَ كَفَرُوا۟ يُحِلُّونَهُۥ عَامًا وَيُحَرِّمُونَهُۥ عَامًا لِّيُوَاطِـُٔوا۟ عِدَّةَ مَا حَرَّمَ ٱللَّهُ فَيُحِلُّوا۟ مَا حَرَّمَ ٱللَّهُ ۚ زُيِّنَ لَهُمْ سُوٓءُ أَعْمَـٰلِهِمْ ۗ وَٱللَّهُ لَا يَهْدِى ٱلْقَوْمَ ٱلْكَـٰفِرِينَ

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.