the Messenger of Allah said: "The most virtuous fasting after the month of Ramadan is Allah's month Al-Muharram."  

قَالَ رَسُولُ اللَّهِ ﷺ أَفْضَلُ الصِّيَامِ بَعْدَ صِيَامِ شَهْرِ رَمَضَانَ شَهْرُ اللَّهِ الْمُحَرَّمُ  

"A man asked Ali: "Which month do you order me to fast after the month of Ramadan?' He said to him, 'I have not heard anyone ask this except for a man whom I heard asking the Messenger of Allah while I was sitting with him. He said: "O Messenger of Allah! Which month do you order me to fast after the month of Ramadan?" He said: "If you will fast after the month of Ramadan, then fast Al-Muharram, for indeed it is Allah's month in which there is a day that Allah accepted the repentance of a people, and in which He accepts the repentance of other people."  

عَنْ هَذَا إِلاَّ رَجُلاً سَمِعْتُهُ يَسْأَلُ رَسُولَ اللَّهِ ﷺ وَأَنَا قَاعِدٌ فَقَالَ يَا رَسُولَ اللَّهِ أَىُّ شَهْرٍ تَأْمُرُنِي أَنْ أَصُومَ بَعْدَ شَهْرِ رَمَضَانَ قَالَ إِنْ كُنْتَ صَائِمًا بَعْدَ شَهْرِ رَمَضَانَ فَصُمِ الْمُحَرَّمَ فَإِنَّهُ شَهْرُ اللَّهِ فِيهِ يَوْمٌ تَابَ اللَّهُ فِيهِ عَلَى قَوْمٍ وَيَتُوبُ فِيهِ عَلَى قَوْمٍ آخَرِينَ