Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) mentioned the day of Ashura and expressed its greatness, and he said to those around him: "Whoever of you has not eaten anything should fast today, and whoever of you has eaten something should fast for the rest of the day."
Al-Tabarani recorded it in al-Mu'jam al-Awsat, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5119