He informed him that the Messenger of Allah ﷺ said to them one day, "This is the day of Ashura, so fast."A man from the tribe of Amr bin Awf said, "Messenger of Allah, I left my people, some fasting and some not."The Prophet ﷺ replied, "Go back to them, and whoever among them has not been fasting should complete the rest of the day as a fast."
حدثنا هشام بن سعيد قال أخبرنا معاوية بن سلام قال سمعت يحيى بن أبي كثير قال أخبرني
أخبره أن رسول الله ﷺ قال لهم يوما هذا يوم عاشوراء فصوموا فقال رجل من بنى عمرو بن عوف يا رسول الله إني تركت قومي منهم صائم ومنهم مفطر فقال النبي ﷺ اذهب إليهم فمن كان منهم مفطرا فليتم صومه
Its chain of transmission is sahih. Bajah bin Abdullah is a well-known reliable narrator whose hadith appears in Sahihayn. This hadith has already come up many times, with its cross-references numbering 26.