Why do Shias beat upon themselves on 9/10 muharram

Re: Why do Shias beat upon themselves on 9/10 muharram

http://www.paklinks.com/gs/religion-and-scripture/371098-khutba-of-zainab-alis-daughter-in-yazids-court.html

This khutbah is recorded by Abu Tahir Tayfur al-Baghdadi, a prolific ninth century historian, in one of his books “Balaghatun Nisa”.

Abu Tahir was the first person to write a history of Baghdad. There is a book on him by Shawkat Toorawa.

“Ibn Abī Ṭāhir Ṭayfūr and Arabic writerly culture: a ninth-century bookman in Baghdad”