It is narrated from Sayyiduna Hind bin Asma' (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) sent me to my people, Banu Aslam, and said: "Tell your people to fast this day, the day of Ashura. And whoever of them you find has eaten something in the first part of the day, let him fast the rest of the day."
Hadith Referenceالمعجم الكبير للطبراني / باب الهاء / 18765