The Messenger of Allah ﷺ sent a man from Aslam on the day of Ashura and instructed him to announce to the people, "Whoever has not yet fasted should fast, and whoever has already eaten should complete his fast until nightfall."
بعث رسول الله ﷺ رجلا من أسلم يوم عاشوراء، فأمره أن يؤذن في الناس: من كان لم يصم فليصم، ومن كان أكل فليتم صيامه إلى الليل.
Bukhari narrated it in the Book of Fasting (1924), and Muslim in the Book of Fasting (1135), through the route of Yazid bin Abu Ubayd, from Salama bin Akwa, who related it. The wording given here is Muslim's.As for his statement, "a man from Aslam," Ibn Hajar said, the name of this man was Hind bin Asma bin Haritha Aslami. He, his father, and his uncle Hind bin Haritha all had the honor of being companions of the Prophet.