Musnad Ahmad 242:2

'Affan narrated to us, he said: Wuhayb narrated to us, 'Abd al-Rahman ibn Harmalah narrated to us, from Yahya ibn Hind ibn Harithah—and Hind was among the companions of al-Hudaybiyyah, and his brother whom the Messenger of Allah, peace and blessings of Allah be upon him, sent to command his people to fast the day of 'Ashura' was Asma' ibn Harithah. So Yahya ibn Hind narrated to me, from Asma' ibn Harithah: That the Messenger of Allah, peace and blessings of Allah be upon him, sent him and said: "Command your people to fast this day." He said: "What if I find that they have already eaten?" He said: "Then let them complete the remainder of their day."