Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) sent a man from the tribe of Aslam to his people on the day of Ashura and said: "Order them to fast this day." He said: O Messenger of Allah! Tell me, what if they have already eaten? He (peace and blessings be upon him) said: "Whoever among them has eaten, let him fast the rest of the day."
Imam al-Bukhari (may Allah have mercy on him) recorded it in his Sahih, and it has been narrated in a hadith that he ordered it to be made up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8035