Historical Rules · 15 July 2011
Rule 605.1
Official source text from the 15 July 2011 Rules release
Some activated abilities and some triggered abilities are mana abilities, which are subject to special rules. Only abilities that meet either of the following two sets of criteria are mana abilities, regardless of what other effects they may generate or what timing restrictions (such as "Activate this ability only any time you could cast an instant") they may have.