Ba'jah bin Abdullah bin Badr narrates that his father told him: One day the Prophet (peace and blessings be upon him) said to the people: "This is the day of Ashura, so fast on it." A man from Banu Amr bin Awf said: "O Messenger of Allah, I left my people in such a state that some of them were fasting and some of them were not fasting." The Prophet (peace and blessings be upon him) said: "Go to them, and whoever of them was not fasting should complete his fast (that is, should not eat or drink anything for the rest of the day)."
Ahmad and al-Tabarani in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat recorded it, and al-Bazzar recorded it; its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5110
Hadith Gradingمحدثین:إسنادہ حسن۔
Hadith Takhrij«اخرجه احمد فى المسند (467/6) ، أخرجه مسند أحمد برقم: 27099، وأخرجه المعجم الأوسط للطبراني برقم: 5679»