Alqama reported,

Ashath bin Qays came in upon Ibn Masud while he was eating on the day of Ashura and said, "Abu Abd al-Rahman, today is the day of Ashura!"Ibn Masud replied, "It used to be fasted before Ramadan was prescribed, but when Ramadan was prescribed, it was abandoned. So if you are not fasting, come and eat."

Azami 4483

Sahih

عن علقمة، قال

دخل الأشعث بن قيس على ابن مسعود وهو يأكل يوم عاشوراء، فقال: يا أبا عبد الرحمن، إن اليوم يوم عاشوراء! فقال: قد كان يصام قبل أن ينزل رمضان، فلما نزل رمضان ترك، فإن كنت مفطرا فاطعم.

الأعظمي 4483صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Fasting
Chapter
Voluntary fasting and encouragement to do it
Section
Clarification on the abrogation of the obligation to fast on Ashura after the obligation of fasting in the month of Ramadan
Numbering
Jami al-Kamil 4483 (2nd edition)
Azami 4483