Section 89 of the Doctrine and Covenants. It forbids the use of liquor, tobacco, and “hot drinks” (which have been officially interpreted as tea and coffee). Breaking the Word of Wisdom is one of the more serious sins in Mormonism. Strictly following it is essential for receiving a temple recommend.
It is common practice for Mormons to enjoy hot drinks such as herbal tea or hot cider. They are technically allowed to drink caffeinated drinks such as soda, but it is frowned upon since part of following the Word of Wisdom is not becoming addicted to any substance.