Historical Rules · 1 February 2013
Rule 505.5b
Official source text from the 1 February 2013 Rules release
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 305, "Lands.") This action doesn't use the stack. Neither the land nor the action of playing the land is a spell or ability, so it can't be countered, and players can't respond to it with instants or activated abilities.