Sunan at-Tirmithiy كتاب الصوم باب ما جاء فى صوم المحرم

حَدَّثَنَا قُتَيْبَةُ حَدَّثَنَا أَبُو عَوَانَةَ عَنْ أَبِى بِشْرٍ عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَن ِ الْحِمْيَرِىِّ عَنْ أَبِى هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَفْضَلُ الصِّيَامِ بَعْدَ صِيَامِ شَهْرِ رَمَضَانَ شَهْرُ اللَّهِ الْمُحَرَّمُ. قَالَ أَبُو عِيسَى حَدِيثُ أَبِى هُرَيْرَةَ حَدِيثٌ حَسَنٌ.
Footnote (1) Muḥarram is the first month of the Islamic calendar. It is one of the four months singled out and made sacred in the Qur’ān. Sins in these months are more serious and good deeds bring a greater reward. (Tafsīr Ibn Kathīr)
The Messenger of God, peace and blessings of God be upon him, said, “The best fast after fasting the month of Ramaḍān is that in God's month of Muḥarram (1).”