Sunan Ibn Majah, Volume 2, Book 7, Hadith 1737 (DarusSalam) ⓘ

It was narrated from ‘Abdullah bin ‘Umar that the Day of ‘Ashura’ was mentioned in the presence of the Messenger of Allah. The Messenger of Allah said: “That was a day when the people of the Ignorance used to fast. So whoever among you wants to fast may do so, and whoever does not want to may leave it.”

حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ، أَنْبَأَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ ذُكِرَ عِنْدَ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ يَوْمُ عَاشُورَاءَ فَقَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ كَانَ يَوْمًا يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَصُومَهُ فَلْيَصُمْهُ وَمَنْ كَرِهَهُ فَلْيَدَعْهُ ‏" ‏ ‏.‏