How do I answer a Sunni brother regarding Matam where he asked if there was any narration of Imam Hussain AS beating his chest on the martyrdom of Imam Ali AS? When did the practice of ritual chest beating start?
Beating the chest is an expression of sadness and grief used by human beings in many societies including Muslims. Ayisha daughter of Abu Bakr did the same when her father died as you can see in Sunni books e.g. Fat-h al-Baari bishar-h Saheeh al-Bukhari by Ibn Hajar al-Asqalani, V.6, P. 225.
Muslims did beat their chests on the demise of the Prophet Muhammad (SAWA) including Aiysha and other Muslim women as she said herself.( Musnad Abi Ya'la al-Mousili , V.8, P. 63.),
Same was done by those who love Ameerul Mo'minnen and other Infallible Imams on their demises. Those who do not have real love for Ahlul Bayt (AS) do not feel sad for the tragedies of Ahlul Bayt (AS) hence do not understand why lovers of Ahlul Bayt (AS) do that. It is in fact an expression of noble feeling of sadness and grief for the tragedies of the Prophet (SAWA) and his Holy Progeny Ahlul Bayt (AS).
We can not help those who do have such feeling for the Prophet (SAWA) and Ahlul Bayt (AS).
Wassalam.