Historical Rules · 13 November 2015
Rule 107.4c
Official source text from the 13 November 2015 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.)