Joseph preaches baptism for the dead at October 1840 GC.
"Minutes of the general conference of the church of Jesus Christ of Latter Day Saints, held in Nauvoo, Hancock county, Ill. Oct., 3rd 1840," Times and Seasons 1, no. 2 (October 1840): 186
President Joseph Smith jr. then arose and delivered a discourse on the subject of baptism for the dead, which was listened to with considerable interest, by the vast multitude assembled.