Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) sent a man from the tribe of Aslam to his people on the day of Ashura, to order them to fast on this day. He said: I do not think I will reach them before they have eaten. He (peace and blessings be upon him) said: "Whoever of them has eaten, let him fast the rest of the day."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8407