The Messenger of Allah ﷺ said on the day of Ashura, "Has any of you eaten today?"They replied, "Some of us have fasted, and some of us have not."He said, "Then complete the rest of your day, and send word to the people of the surrounding regions to complete the rest of their day as well."
أخبرنا عبد الله بن أحمد بن عبد الله بن يونس أبو حصين قال حدثنا عبثر قال حدثنا حصين عن الشعبي
قال رسول الله ﷺ يوم عاشوراء أمنكم أحد أكل اليوم فقالوا منا من صام ومنا من لم يصم قال فأتموا بقية يومكم وابعثوا إلى أهل العروض فليتموا بقية يومهم
[Its isnad is sahih] Ibn Majah narrated it in Kitab as-Siyam, chapter on fasting the day of Ashura, no. 1735, from the hadith of Husayn, in this manner, and it is also in Al-Kubra, no. 2629. Ibn Khuzayma graded it sahih (3/289, no. 2091), as did Ibn Hibban (no. 932), and Busiri.