Ibn Sindar reported,

Some men reported that the Messenger of Allah ﷺ said to a man from Aslam after morning had broken on the Day of Ashura, "Go to your people and tell them to fast this day."The man from Aslam asked, "Messenger of Allah, what if I find someone among them who has already eaten?"The Messenger of Allah ﷺ replied, "Let him complete his fast."

Azami 4474

Hasan

عن ابن سندر

عن رجال منهم أن رسول الله ﷺ قال لرجل من أسلم من بعد ما أصبح من يوم عاشوراء: اذهب إلى قومك فمرهم فليصوموا هذا اليوم. قال الأسلمي: يا رسول الله أرأيت إن وجدت أحدا منهم تغدى؟ قال رسول الله ﷺ: فليتم صومه

الأعظمي 4474حسن
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Fasting
Chapter
Voluntary fasting and encouragement to do it
Section
Emphasis on the obligation of fasting on Ashura
Numbering
Jami al-Kamil 4474 (2nd edition)
Azami 4474