Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Fast on the day of Ashura and differ from the Jews: fast one day before it or one day after it."
(In the narration of Ibn Abdan, he (peace and blessings be upon him) said: "Fast one day before it and one day after it.")
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8406