Rav Feinstein and the Alter Rebbe of Chabbad

So I was listening to this dvar torah

http://collive.com/show_news.rtx?id=22947&alias=rav-feinsteins-debt-to-chabad

where the actual story about Rav Feinstein and the Alter Rebbe starts 13minutes in. He says the story is found in the 8th volume of Igros Moshe in the introduction.

Could someone help me find it? Because I can’t seem to find it.

thanks