Why do Shia Muslims perform Matam or chest-beating? Did any of the children or descendants of Imam Ali (AS) ever perform it? If not, is there a valid reason, with references, that explains the origin and justification of this practice?
Chest beating is an action to express grief and sadness. It goes back to centuries as we read in Hadeeth books that Aysha daughter of Abu Bakr used to beat her face and chest (Dala'il al-Nobowwah by Al-Bayhaqi V.7, P.213 and Musnad Ahmad Ibn Hanbal V.43, P. 369.
Wassalam.