Fasting on the day of ‘Ashura and Ramadan

عَنِ ابْنِ عُمَرَ رضى اللّٰہ عنهما قَالَ: صَامَ النَّبِيُّ صلّى اللّٰہ عليه وسلّم عَاشُورَاءَ، وَأَمَرَ بِصِيَامِهِ‏.‏ فَلَمَّا فُرِضَ رَمَضَانُ تُرِكَ‏.‏ وَكَانَ عَبْدُ اللَّهِ لاَ يَصُومُهُ، إِلاَّ أَنْ يُوَافِقَ صَوْمَهُ‏.‏

Hazrat Ibn Umar^ra narrated: 

“The Prophet^sa fasted on ‘Ashura [the tenth of Muharram] and instructed the people to fast it. Then, when the fasting of Ramadan was made obligatory, it was left. Hazrat ‘Abdullah^ra [Ibn ‘Umar^ra] would not fast it, unless it happened to coincide with his usual fasting [routine].”

(Sahih al-Bukhari, Kitab as-sawm, Bab wujubi sawmi ramadan, Hadith 1892)