According to an 1890 biography, Wendell Phillips: The Agitator..., page 446, Phillips attributed the line to John Brown. Phillips used to include it as part of his autograph; you can find some examples for sale on collectors' sites (such as the one above).
Brown, if he did in fact say or write it, was apparently adapting Martin Luther's quote, often translated as "Peace if possible, truth at all costs." (see this search result list for a variety of attributions)
Perhaps someone else can come up with "chapter and verse" sources for Brown and Luther?