Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Ashura is the day of Eid of a Prophet who came before you, so you should fast on it."
Al-Bazzar recorded it. Its chain contains a narrator, Ibrahim al-Hajari, whom Ibn Adi declared thiqah (trustworthy), while the Imams called him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5115