Sahih Muslim | Book 6 | Hadith Number 2530

Narrated by Salama b. al-Akwa' (Allah be pleased with him)
Salama b. al-Akwa' (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) sent a person of the tribe of Aslam on the day of 'Ashura and commanded him to declare to the people to observe fast in case they had not observed it, and to complete fast till evening if they had taken food