It is narrated from Sayyiduna Khabbab (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said on the day of Ashura: "O people! Whoever among you wishes to fast this day, let him fast it; whoever has not yet eaten anything, let him complete his fast; and whoever has eaten, let him fast the rest of the day."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3676