Dawud bin Ali (may Allah have mercy on him) narrates from his father, from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "If I remain, I will order fasting a day before or a day after the day of Ashura."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8405