Here's the relevant rules:
305.4. During either main phase, the active player may play one land card from his or her hand if the stack is empty, if the player has priority, and if he or she hasn’t yet taken this special action this turn. (See rule 212.6, “Lands.”) This action doesn’t use the stack and it isn’t a spell or ability of any kind. It can’t be countered, and players can’t respond to it with instants or activated abilities.
Also, here's a link to download the latest
rules