As far as I know, it wasn't congressional testimony.
It was in an April 18, 1971, appearance on Meet The Press. I don't have the original transcript.
Kerry said, "there are all kinds of atrocities and I would have to say that, yes, yes, I committed the same kind of atrocities as thousands of other soldiers have committed in that I took part in shootings in free-fire zones. I conducted harassment and interdiction fire. I used 50-caliber machine guns which we were granted and ordered to use, which were our only weapon against people. I took part in search-and-destroy missions, in the burning of villages. All of this is contrary to the laws of warfare. All of this is contrary to the Geneva Conventions and all of this ordered as a matter of written established policy by the government of the United States from the top down."
I've heard the actual tape of this any number of times.