Searching for the "Last shiur in the Holocaust: Shatnez"

I am hoping someone here will have heard of this story and will be able to share more details and also the contents of the lecture.

I once heard that a Rabbi was with a group of men, about to be taken to the their death, and the Rabbi chose the topic of his last shiur to be on the laws of shatnez, and he went on to explain how this connects to all/many other mitzvots and it was the most important concept to understand.

While I arrived at the same conclusions when I was still a very secular Jew, I would love to finally read a shiur on this subject from a Rabbi.

If anyone has ever heard of this story and has any details to complete this story it would help me tremendously.

Toda-