Musnad Ahmad 333:16

Hammad ibn Mas'adah narrated to us, from Yazid—meaning Ibn Abi 'Ubayd—from Salamah ibn al-Akwa', "That the Prophet, peace and blessings of Allah be upon him, ordered a man from Aslam to announce to the people on the day of 'Ashura: 'Whoever is fasting, let him complete his fast, and whoever has eaten, let him not eat anything, and let him complete his fast.'"