Who said a man is innocent until proven guilty? When was this said?
In terms of American History, one can see the presence of this principle in the forming of the Constitution. The Sixth Amendment presents the details within which criminal trials should operate. One of these premises is the notion that a fundamental right of the accused is to be perceived as "innocent until proven guilty" and that all those accused of a crime shall have this idea as a part of their criminal proceedings. This becomes the burden that all judges, juries, as well as legal counsel must carry. It is this idea that maintains a sense of fairness within the legal framework outlined by the Constitution and ensures that justice can be appropriately meted out by the legal system it outlines.
It is not really known who first said this. It's not one of those famous quotes from history that has a clear author. The idea has been around a long time -- the idea that someone is assumed to be innocent.
The phrase itself, according to the article that I've linked to below, can first be found in the French Declaration of the Rights of Man from the French Revolution in 1789.
We know for sure that the phrase was first used in American law in 1894 when it was used in a Supreme Court decision.