It is narrated from 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 should complete his fast, and whoever is not fasting should also complete the rest of the day (like one who is fasting)."
Imam al-Tabarani (may Allah have mercy on him) said: "No one other than Sharik (may Allah have mercy on him) narrated this hadith from Majza'ah (may Allah have mercy on him)."
Hadith Referenceالمعجم الأوسط / باب الألف / 589
Hadith Takhrij«أخرجه ابن حجر فى «المطالب العالية» برقم: 1081، وابن أبي شيبة فى «مصنفه» برقم: 9445، والطحاوي فى «شرح مشكل الآثار» برقم: 2276، والطبراني فى «الكبير» برقم: 5312»