It is narrated from Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) that on the day of Ashura, the Messenger of Allah (peace and blessings be upon him) sent out a caller who was announcing: "Whoever has not fasted should complete the rest of his day (as if fasting), and whoever has fasted should complete his fast."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6261