Abdullah bin Masud reported,

Ashath came in while he was eating and said, "Today is Ashura."He replied, "It used to be fasted before Ramadan was prescribed. When Ramadan was prescribed, it was abandoned. Come closer and eat."

Bukhari 4503

Sahih

حدثني محمود أخبرنا عبيد الله عن إسرائيل عن منصور عن إبراهيم عن علقمة

عن عبد الله قال

دخل عليه الأشعث وهو يطعم فقال اليوم عاشوراء فقال كان يصام قبل أن ينزل رمضان فلما نزل رمضان ترك فادن فكل

البخاري 4503صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Tafsir
Chapter
O you who believe, fasting has been prescribed for you as it was prescribed for those before you, that you may attain taqwa
Numbering
Darussalam 4503
Bukhari 4503