Silsilat al-Ahadith al-Sahihah

The Messenger of Allah (peace and blessings be upon him) said: "On this day of Ashura, announce among your people or among the people that whoever has eaten should observe fasting for the rest of the day, and whoever has not eaten should fast for the whole day." This hadith is narrated from Salamah bin Akwa’, Rabi’ bint Mu’awwidh, Muhammad bin Saifi, Hind bint Asma, Abu Hurairah, and Abdullah bin Abbas (may Allah be pleased with them), from some unspecified individuals of the Aslam tribe, from Ma’bad Qurashi, and is also narrated mursal from Muhammad bin Sirin.

It is narrated from Nafi‘ that Sayyiduna ‘Abdullah bin ‘Umar (may Allah be pleased with them both) related to him that he heard the Messenger of Allah (peace and blessings be upon him) say on the Day of ‘Ashura: “The people of the Age of Ignorance used to fast on this day. Now, whoever wishes to fast on it, let him fast, and whoever wishes to leave it, let him leave it.”

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the people of the Age of Ignorance used to fast on the Day of Ashura (that is, the tenth of Muharram), and before the fasting of Ramadan was made obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) and other Muslims also used to fast. When Ramadan was made obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Ashura is one of Allah’s days; whoever wishes may fast on it, and whoever wishes may leave it."