I recently came across this sermon from Nahjulbalagha and it states that Imam Ali said that Prophet Muhammad had prevented them from wailing and mourning. Why do we mourn then?

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Wailing is recommended for Imam Hussain (as) in many narrations.

When we have a loss in our lives we remember and mourn Imam.
I highly recommend this book

Commemorating the Mourning Rights of Imam al-Husayn https://smile.amazon.com/dp/1514298619/ref=cm_sw_r_cp_api_glt_fabc_25H7R9JH0X7R70JBFGW6

https://youtu.be/Fdt19d8E90Q