Abdur-Rahman bin Yazid (may Allah have mercy on him) narrates that Sayyiduna al-Ash'ath bin Qays (may Allah be pleased with him) came to Sayyiduna Abdullah (may Allah be pleased with him) on the day of Ashura while he was eating. He said: O Abu Muhammad! Come near and eat. He said: Is today not the day of Ashura? He said: Do you know what the day of Ashura is? It was only a day on which the Messenger of Allah (peace and blessings be upon him) used to fast before (the fasting of) Ramadan was revealed; then when Ramadan was revealed, he (peace and blessings be upon him) left it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8411