Musnad Ahmad 333:34

Yahya ibn Sa'id narrated to us, from Yazid ibn Abi 'Ubayd, who said: Salamah ibn al-Akwa' narrated to us that the Messenger of Allah (peace and blessings of Allah be upon him) said to a man from Aslam: "Announce among your people, or among the people, on the day of 'Ashura': Whoever has eaten, let him fast the remainder of his day, and whoever has not eaten, let him fast."