Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answered 3 months ago

It is not obligatory but it is recommended. It is considered to be an appropriate form of expression on Ashura, and a means of attracting divine blessings and linking the followers of the Ahl al-Bayt to the Ahl al-Bayt.

Here are some hadith that may be helpful: https://www.al-islam.org/forty-hadith-on-azadari/ahadith-traditions