Historical Rules · 1 May 2011
Rule 202.1b
Official source text from the 1 May 2011 Rules release
Some objects have no mana cost. This normally includes all land cards, any other cards that have no mana symbols where their mana cost would appear, tokens (unless the effect that creates them specifies otherwise), and nontraditional _Magic_ cards. Having no mana cost represents an unpayable cost (see rule 117.6). Note that lands are played without paying any costs (see rule 305, "Lands").