Majza'ah bin Zahir relates from his father, Sayyiduna Zahir (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) ordered the fast of Ashura' and said: "Whoever is fasting, let him complete his fast, and whoever is not fasting, let him complete the rest of his day (without eating)."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5293