Humayd bin Abdur-Rahman bin Awf (may Allah have mercy on him) narrates that he heard Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) on the day of Ashura, in the year he performed Hajj, saying while he was on the pulpit: O people of Madinah, where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) saying: "This day is the day of Ashura. Allah Almighty has not made its fasting obligatory upon you; whoever wishes may fast on it, and whoever wishes may not."
(Al-Shafi'i added in his narration: "And I am fasting, so whoever wishes, let him fast," and the rest of the narration has the same meaning. His saying "Allah has not made its fasting obligatory upon you" indicates that it was never obligatory, because 'lam' refers to the past.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8416
Humayd (may Allah have mercy on him) narrates that Sayyiduna Mu'awiyah (may Allah be pleased with him) said on the pulpit of Madinah: O people of Madinah! Where are your scholars? The Messenger of Allah (peace and blessings be upon him) used to forbid the like of this. And he took out a lock of hair from his sleeve and said: The Children of Israel were destroyed only when their women adopted the like of this. O people of Madinah! Where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) on this day (the day of Ashura) saying: "I am fasting; whoever of you wishes to fast, let him fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8417
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Messenger of Allah (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The day of Ashura is a day on which the people of the Days of Ignorance (Jahiliyyah) used to fast. So whoever of you likes to fast on it, let him fast, and whoever does not like it, let him leave it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8418
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrated to them that he heard the Messenger of Allah (peace and blessings be upon him) saying about the day of Ashura: "This is a day on which the people of the Days of Ignorance (Jahiliyyah) used to fast. Whoever of you likes, let him fast on it, and whoever does not wish, let him not fast."
(And Abdullah (may Allah be pleased with him) would not fast on it unless it coincided with his (regular) fasting.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8419
Sayyidah Aishah (may Allah be pleased with her) narrates that the people of Quraysh used to fast on the day of Ashura in the Days of Ignorance (Jahiliyyah). When Islam came, the Messenger of Allah (peace and blessings be upon him) said: "Whoever wishes may fast on it, and whoever wishes may not."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8420