The Messenger of Allah ﷺ came out to us on the day of Ashura and asked, "Have you fasted today?"Some said, "Yes," and others said, "No."He said, "Then complete the rest of your day in fasting." He instructed them to send word to the people of the surrounding regions to complete the remainder of that day fasting as well.
حدثنا هشيم أخبرنا حصين عن الشعبي
خرج علينا رسول الله ﷺ في يوم عاشوراء فقال أصمتم يومكم هذا فقال بعضهم نعم وقال بعضهم لا قال فأتموا بقية يومكم هذا وأمرهم أن يؤذنوا أهل العروض أن يتموا يومهم ذلك
No references are available.
Its isnad is sahih, its narrators are trustworthy and well known, and the hadith was narrated in similar wording by Bukhari 4/245, no. 2007 (Fath), Muslim 2/792, no. 1129, Abu Dawud 2/327, no. 2447, and Nasai 4/192, no. 2320, all of them in the chapter on fasting, on fasting Ashura.