World Premier recording from April 24th 1976: From Steve Reich's official website you can find a recording from the world premier on April 24th 1976 at Town Hall, New York City by Steve Reich and Musicians. This is just the first 12 minutes of the piece, from the opening pulses through to section II. The recording available on the internet is limited to 64kbps so not wonderful quality but worth listening to from the historical perspective. The link is here.
European premier: From the same era: A recording has surfaced on Youtube of a recording from Paris on October 19th 1976. This would have presumably been the same musicians as appeared on the original ECM recording. It is broken down into 4 individual videos sections, and is an interesting listen. Here is part 1.