The Messenger of Allah ﷺ said to them one day, "This is the Day of Ashura, so fast it."A man from the tribe of Amr bin Awf said, "Messenger of Allah, I left my people behind, some of them are fasting and some are not."The Prophet ﷺ said, "Go back to them, and whoever among them has not been fasting, let him complete the rest of his day as a fast."
أن رسول الله ﷺ قال لهم يوما: هذا يوم عاشوراء، فصوموه. فقال رجل من بني عمرو بن عوف: يا رسول، إني تركت قومي، منهم صائم، ومنهم مفطر. فقال النبي ﷺ: اذهب إليهم، فمن كان منهم مفطرا فليتم صومه.
Narrated by Ahmad (27646), Bazzar - Kashf Astar (1049), and Tabarani in Awsat (5679), all from the hadith of Muawiya bin Salam, who said, "I heard Yahya bin Abu Kathir say, 'Bujja bin Abdullah informed me that his father told him,'" and he related it. Its isnad is sahih. Yahya bin Abu Kathir is trustworthy and firmly reliable, except that he practices tadlis and irsal, but here he explicitly stated the mode of transmission. Haythami graded it hasan in Majma (3/185).