It is narrated from the father of Ba’jah, Sayyiduna Abdullah (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said to him one day: Today is the day of Ashura, observe fasting on this day. A man from Banu Amr bin Awf said: O Messenger of Allah! I have left my people in such a state that some of them are fasting and some are not. The Prophet (peace and blessings be upon him) said: Go to them and tell them that whoever has not been fasting should observe fasting for the rest of the day.
Hadith Referenceالفتح الربانی / كتاب الصيام / 3914