While discussing the virtue and prescribed acts of worship legislated for the day of ‘aashooraa’ (the 10th of Muharram), Ibn Rajab al-Hanbali briefly yet eloquently addresses a common misconception. He writes:
وأما اتخاذه مأتماً كما تفعله الرافضة لأجل قتل الحسين بن علي رضي الله عنهما فيه, فهو من عمل مَن ضلّ سعيه في الحياة الدنيا وهو يحسب أنّه يُحسن صُنعاً, ولم يأمر الله ولا رسولُه باتخاذ أيّام مصائب الأنبياء وموتهم مأتماً, فكيف بمن دُونَهم؟
And as for taking it [i.e.yawm al-‘aashooraa’] as a time of mourning as the raafidhah (Shi’a) have made it on account of the murder of al-Hussain ibn ‘Ali (radiAllaahu ‘anhuma) on this day, then this is from the actions of those whose efforts have gone astray in the life of this dunya and who consider it as a beautified invention. And neither Allah nor His messenger ordered the taking of the calamitous days of the anbiyaa’ or their deaths as times of mourning, so then what about someone less than them?
[Lataa’if al-Ma’aarif pg. 113]
For a explanation of the true origin of fasting ‘Aashooraa’ and its merits, see: Khutbah regarding ‘Aashooraa’: Imam al-Sa’di
See also: Fasting ‘Aashooraa’: Sharh al-Nawawi