Musnad Ahmad 242:1

Ya'qub ibn Ibrahim narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: 'Abdullah ibn Abi Bakr ibn Muhammad narrated to me, from Habib ibn Hind ibn Asma' al-Aslami, from Hind ibn Asma', he said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me to my people from Aslam and said: "Command your people to fast this day, the day of Ashura. Whoever among them you find who has eaten at the beginning of his day, let him fast the remainder of it."