Historical Rules · 10 August 2018
Rule 202.1b
Official source text from the 10 August 2018 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”).