After talking to David Whitmer on 13 April 1882, Matthias Cowley wrote:
During his remarks he said in substance that the Book of Mormon contained all that is necessary to guide us till the Savior comes. I was impressed with the idea, or fact, that he stood in the same position relative to the Book of Mormon that the sectarian world did relative to the Bible.
(As Kenneth W. Godfrey summarizes from the same diary entry: "Matthias F. Cowley, after talking with David Whitmer, was impressed that he [Whitmer] stood in the same position to the Book of Mormon as the sectarians did to the Bible. Whitmer told him that the 'Book of Mormon contained all that is necessary to guide us till the Savior comes.'")
The original Matthias F. Cowley diary is held in manuscript at the Church History Library (CHL), Salt Lake City, and the full diary entry is not currently digitized for open public access; the substantive content of the 13 April 1882 entry as it bears on David Whitmer is preserved verbatim in the published quotations above (drawn from the standard scholarly transcriptions in Cook, David Whitmer Interviews, 91, and in Godfrey, "David Whitmer and the Shaping of Latter-day Saint History"). The opening logistical line ("Thursday, 13th, President Morgan arrived with KPRR train, the AT and SF train being behind time") is preserved separately in the Cook compilation as the diarist's notation of Morgan's arrival in Kansas City; the substantive Whitmer-related content of the entry is the passage quoted above.