Chapter: Fasting is obligatory in (the month of) Ramadan
Narrated Ibn `Umar:The Prophet (ﷺ) observed the fast on the 10th of Muharram ('Ashura), and ordered (Muslims) to fast on that day, but when the fasting of the month of Ramadan was prescribed, the fasting of the 'Ashura' was abandoned. `Abdullah did not use to fast on that day unless it coincided with his routine fasting by chance.
Reference
Sahih al-Bukhari 1892
In-book reference
Book 30, Hadith 2
USC-MSA web (English)
Vol. 3, Book 31, Hadith 116
Translation by Dr. M. Muhsin Khan · Language: EN
Sahih
Arabic Text
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا إِسْمَاعِيلُ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ صَامَ النَّبِيُّ صلى الله عليه وسلم عَاشُورَاءَ، وَأَمَرَ بِصِيَامِهِ. فَلَمَّا فُرِضَ رَمَضَانُ تُرِكَ. وَكَانَ عَبْدُ اللَّهِ لاَ يَصُومُهُ، إِلاَّ أَنْ يُوَافِقَ صَوْمَهُ.