Reuben Miller records Oliver Cowdery stating how Joseph translated the BOM and Rigdon did not make it up.
Reuben Miller, Journal, October 21, 1848, microfilm of holograph, MS 1392, Church History Library
Friends and brethren my name is Cowdrey, Oliver Cowdrey, In the early history of this church I stood Identified with her. And [was] one in her councils. . . .I wrote with my own pen the intire book of mormon (Save a few pages) as it fell from the Lips of the prophet [Joseph Smith]. As he translated <it> by the gift and power of god, By [the] means of the urum and thummim, or as it is called by that book holy Interperters. I beheld with my eyes. And handled with my hands the gold plates from which it was translated. I also beheld the Interperters. That book is true. Sidney Rigdon did not write it. Mr [Solomon] Spaulding did not write it. I wrote it myself as it fell from the Lips of the prophet.