Which we known as Codex Sinaiticus. At the British Museum:
… One of the highlights is part of the 4th century Codex Sinaiticus, a book written in Greek on animal skin by monks on Mount Sinai, and which contains the oldest complete copy of the New Testament.
“It is without question the most important book in Britain. It is a remarkable chance to see it in the context of the world in which it was made,” said British Museum director Neil MacGregor…
More info here.