From the Doctrine and Covenants 89:17
17 Nevertheless, wheat for man, and corn for the ox, and oats for the horse, and rye for the fowls and for swine, and for all beasts of the field, and barley for all useful animals, and for mild drinks, as also other grain.
Doctrine and Covenants 89
From Wikipedia:
The production of beer is called brewing, which involves the fermentation of starches, mainly derived from cereal grains—most commonly malted barley.
Beer – Wikipedia
Crash Course:
Word of Wisdom – Wikipedia
Word of Wisdom – Mormon Think
Doctrine and Covenants 89
Evolution of the Word of Wisdom – The Origin Story – wasmormon.org
Evolution of the Word of Wisdom – Stories we Tell – wasmormon.org
Evolution of the Word of Wisdom – Barley Drinks – wasmormon.org