The Excellence of Observing Saum on the Day of 'Arafah, 'Ashura' and Tasu'a (i.e., 9th of Muharram)
باب فضل صوم يوم عرفة وعاشوراء وتاسوعاء:
This chapter draws together the most rewarding single days to fast outside Ramadan: the Day of 'Arafah (9th of Dhul-Hijjah), 'Ashura' (10th of Muharram), and Tasu'a (the 9th of Muharram). What unites them is the theme of expiation, fasting these days as a means by which God wipes away sins.
Notice the careful gradation across the reports. 'Arafah carries a larger promise than 'Ashura', and the Prophet's wish to fast the ninth alongside the tenth teaches an attentiveness to how we worship, not only that we worship. The forgiveness mentioned here concerns minor sins; major sins call for sincere repentance, a detail the scholars are careful to preserve.
Hadith 1250
Abu Qatadah (May Allah be pleased with him) reported:
وعن أبي قتادة رضي الله عنه قال: سئل رسول الله صلى الله عليه وسلم: عن صوم يوم عرفة؟ قال: "يكفر السنة الماضية والباقية" ((رواه مسلم)).
The Messenger of Allah (ﷺ) was asked about the observance of Saum (fasting) on the day of 'Arafah. He said, "It is an expiation for the sins of the preceding year and the current year." [Muslim].
In plain words
Fasting the Day of Arafah is described as wiping away the sins of the year before and the year to come. A great reward for a single day.
Abu Qatadah (May Allah be pleased with him) reported:
وعن أبي قتادة رضي الله عنه أن رسول الله صلى الله عليه وسلم سئل عن صيام يوم عاشوراء فقال: "يكفر السنة الماضية" ((رواه مسلم)).
The Messenger of Allah (ﷺ) was asked about observing As-Saum (the fast) on the tenth day of Muharram, and he replied, "It is an expiation for the sins of the preceding year." [Muslim].
In plain words
Fasting on the tenth of Muharram, the day of Ashura, is described as wiping away the sins of the preceding year.