where is rfc that defines signature separator "-- \n"?

        Hi,

        This is not directly procmail question, but I'm trying to
        locate the rfc that mentions that the sig. sep. should be
        "-- \n". I visited

        http://www.cis.ohio-state.edu/htbin/rfc

        Which has rfc archive, but it was like trying to find needle
        from haystack. Can anyone recommend good search keywords
        or direct pointer to rfc number?

        jari