Sunan at-Tirmithiy كتاب الصوم باب ما جاء فى صوم المحرم
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) Footnotes: 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).” Click on ḥadīth to read more
A man asked the Messenger of God, peace and blessings of God be upon him, "Which month do you instruct me to fast after the month of Ramaḍān?" He (the Messenger of God) replied, "If you are to fast beyond the month of Ramaḍān, then fast [in the month of] Muḥarram, for it is indeed the month of God; it includes a day in which God has forgiven a people (1), and in which he forgives other people (2)." Footnotes: (1) The Prophet, peace and blessings of God be upon him, is referring to the followers of prophet Moses, peace be upon him, whom God forgave on the tenth day of Muḥarram (the first month of the Islamic calendar) and saved them on that day from their enemy, Pharaoh. Scholars say the Prophet, peace and blessings of God be upon him may have also meant prophet Ādam or the followers of prophet Jonah, who were also forgiven on the same day. (2) God will continue to forgive more people or nations on every annual occurrence of that day. That day is the tenth of Muḥarram; the Arabic name for the holiday is 'Āshūra'. Click on ḥadīth to read more