Yahya bin Hind narrates from his father, Sayyiduna Hind bin Harithah (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) sent him (to make an announcement) on the day of Ashura and said: "Order your people to fast this day." He said: O Messenger of Allah! If I find that they have already eaten, (what is the ruling)? He (peace and blessings be upon him) said: "Then let them complete the rest of their day (like one who is fasting)." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.