Hadith 783:1
Hushaym narrated to us, Husayn informed us, from al-Sha’bi, from Muhammad ibn Sayfi al-Ansari, who said: The Messenger of Allah (may Allah bless him and grant him peace) came out to us on the day of Ashura and said: "Have you fasted this day of yours?" Some of them said: "Yes," and some of them said: "No." He said: "Then complete the remainder of this day of yours." And he commanded them to notify the people of the outskirts to complete that day of theirs.
حدثنا هشيم، أخبرنا حصين، عن الشعبي، عن محمد بن صيفي الأنصارى، قال: خرج علينا رسول الله صلى الله عليه وسلم 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