Mishkat al-Masabih 7 Fasting كتاب الصوم 82

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَفْضَلُ الصِّيَامِ بَعْدَ رَمَضَانَ شَهْرُ اللَّهِ الْمُحَرَّمِ وَأَفْضَلُ الصَّلَاةِ بَعْدَ الْفَرِيضَةِ صَلَاةُ اللَّيْلِ» . رَوَاهُ مُسلم

Abu Huraira reported God’s messenger as saying, “The most excellent fast after Ramadan is in God’s month al-Muharram, and the most excellent prayer after what is prescribed is prayer during the night.” Muslim transmitted it.