On August 6, 1842, Joseph Smith wrote: "I had a conversation with a number of brethren in the shade of the building on the subject of our persecutions in Missouri and the constant annoyance which has followed us since we were driven from that state. I prophesied that the Saints would continue to suffer much affliction and would be driven to the Rocky Mountains, many would apostatize, others would be put to death by our persecutors or lose their lives in consequence of exposure to disease, and some of you will live to go and assist in making settlements and build cities and see the Saints become a mighty people in the midst of the Rocky Mountains."
There are also recollections by other church members of Joseph mapping the path the pioneers would take to the Salt Lake Valley. For example, George H. Goddard, a Mormon Pioneer, testified that he was present in the Nauvoo Masonic Hall when Joseph Smith drew a map on the floor with a piece of chalk the Great Basin of western America, indicating the course they would follow across the plains and into the West.
Another church member, Hopkins C. Pendar, reported that "Joseph Smith just before he was killed, made a sketch of the future home of the Saints in the Rocky Mountains and their route or road to that country as he had seen [it] in vision; a map or drawing of it."
Mosiah Hancock wrote in his journal about Joseph Smith visiting his father, Levi's, home, just prior to his demise in Carthage. Mosiah writes, ". . . the Prophet came to our home and stopped in our carpenter shop and stood by the turning lathe I went and got my map for him. 'Now,' said he, 'I will show you the travels of this people.' He then showed our travels thru Iowa, and said, 'Here you will make a place for the winter; and here you will travel west until you come to the valley of the Great Salt Lake! You will build cities to the North and to the South, and to the East and to the West; and you will become a great and wealthy people in that land.'
Wilford Woodruff, Conference Report, April 1898, p.57
Teachings of the prophet Joseph Smith (Aug. 6, 1842.) (Millennial Star, Vol. xix., page 630)
"The Life Story of Mosiah Lyman Hancock"
E. Cecil McGavin, "Nauvoo the Beautiful" (Salt Lake City, 1946)
Hyrum L. Andrus, BYU Studies, Vol. 2, No. 2, p.141