Historical Rules · 18 June 2010
Rule 107.4c
Official source text from the 18 June 2010 Rules release
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads "add [mana symbol] to your mana pool" or something similar. (See rule 107.3e.)