Oliver Cowdery records a statement on slavery in 1835 Doctrine & Covenants 102 (now 134).
Appendix 4: Declaration on Government and Law, 254, The Joseph Smith Papers website, accessed August 11, 2021
We do not believe it right to interfere with bond-servants, neither preach the gospel to, nor baptize them, contrary to the will and wish of their masters, nor to meddle with, or influence them in the least to cause them to be dissatisfied with their situations in this life, thereby jeopardizing the lives of men: such interference we believe to be unlawful and unjust, and dangerous to the peace of every Government allowing human beings to be held in servitude.