(21) باب مَنْ أَكَلَ فِي عَاشُورَاءَ فَلْيَكُفَّ بَقِيَّةَ يَوْمِهِ
Salama b. al-Akwa' (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) 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
Reference: Sahih Muslim 1135
In-book reference: Book 13, Hadith 174
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2530