Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “The best prayer after the prescribed prayers is a prayer offered in the middle of the night. The best fast after the month of Ramadan is fasting in the month of Allah, al-Muharram.”
Source: Ṣaḥīḥ Muslim 1163
Grade: Sahih (authentic) according to Muslim
عَنْ أَبِي هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَفْضَلُ الصَّلاَةِ بَعْدَ الصَّلاَةِ الْمَكْتُوبَةِ الصَّلاَةُ فِي جَوْفِ اللَّيْلِ وَأَفْضَلُ الصِّيَامِ بَعْدَ شَهْرِ رَمَضَانَ صِيَامُ شَهْرِ اللَّهِ الْمُحَرَّمِ
صحيح مسلم كتاب الصيام أفضل الصيام بعد رمضان شهر الله المحرم 1163