From: Rabbi Moshe Sofer
To: Rabbi Avraham David Rubinstein
The Chasam Sofer analyzes whether the custom of reciting a Yotzros piyyut before Birkas HaShachar on Tisha B'Av has halachic support, and resolves difficulties in the Gemara regarding mourning practices during this period.
Source: Shu"t Chasam Sofer OC §158