Source, in full
The source is likely to be either modern Buddhist monk Thich Nhat Hanh, or Calvinist clergyman Abraham Johannes Muste. The phrase appears in Thich Nhat Hanh's writings; but it also appears in a volume of US senate hearings from 1948, when Thich Nhat Hanh had not yet been ordained as a monk. Muste is known to have used a variant of the phrase – "'peace' is the way" in 1967, but this was not the first time he had used it, and he had a connection with the 1948 hearing.