Ibn 'Umar said:'Ashurah was a day on which we used to fast in pre-Islamic days. When (fasting of) Ramadan was prescribed, the Messenger of Allah (ﷺ) said: This is one of the days of Allah ; he who wishes may fast on it.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ أَخْبَرَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ عَاشُورَاءُ يَوْمًا نَصُومُهُ فِي الْجَاهِلِيَّةِ فَلَمَّا نَزَلَ رَمَضَانُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " هَذَا يَوْمٌ مِنْ أَيَّامِ اللَّهِ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ " .
AbuDaud
Fasting (Kitab Al-Siyam) • Hadith 2443