Mike Leghorn wrote:

>Could it be his piano concerto?

I thought the correct works were already specified in a previous post.
Well this should resolve the question of what work it is:

   http://us.imdb.com/title/tt0120663/fullcredits

Just scroll down to non-original music and you will see Ligeti billed
with the piano music "No.2: Mesto, Rigido e Cerimonale" from "Musica
ricercata" 1950

David Whitbeck