The man who proposed this is named David Wilmot. Wilmot was a Congressman during the time when the US received Arizona, California, New Mexico, and parts of Utah/Colorado from Mexico after the Mexican-American War. Wilmot's goal was to not let the institution of slavery expand any further. His suggestion to ban slavery gained by these new territories was known as the Wilmot Proviso. This never becomes law, as it is passed by the House of Representatives but is never passed by the US Senate.