In the course of the work of translation, we ascertained that three special witnesses were to be provided by the Lord, to whom He would grant that they should see the plates from which this work (the Book of Mormon) should be translated; and that these witnesses should bear record of the same, as will be found recorded, … [ Book of Ether, chapter 5, verses 2, 3 and 4 … ], … [ II Nephi, chapter 11, verse 3 … ].
Almost immediately after we had made this discovery, it occurred to Oliver Cowdery, David Whitmer and the aforementioned Martin Harris (who had come to inquire after our progress in the work) that they would have me inquire of the Lord to know if they might not obtain of him the privilege to be these three special witnesses; and finally they became so very solicitous, and urged me so much to inquire that at length I complied; and through the Urim and Thummim, I obtained of the Lord for them [the revelation recorded in D&C 17].
(Joseph Smith, History of the Church, 1:52–53)