McLellin is quoted in the SLT about Joseph and Alger's sealing.
William McLellin quoted by J. H. Beadle, “Jackson County,” The Salt Lake Tribune 9, no. 147 (October 6, 1875): 4
He [McLellin] also informed me of the spot where the first well authenticated case of polygamy took place, in which Joseph Smith was “sealed” to the hired girl. The “sealing” took place in a barn on the hay mow, and was witnessed by Mrs. Smith through a crack in the door! The Doctor was so distressed about this case, (it created some scandal at the time among the Saints,) that long afterwards when he visited Mrs. Emma Smith at Nauvoo, he charged her as she hoped for salvation to tell him the truth about it. And she then and there declared on her honor that it was a fact—“saw it with her own eyes."