Historical Rules · 3 May 2019
Rule 605.5b
Official source text from the 3 May 2019 Rules release
A spell can never be a mana ability, even if it could put mana into a player’s mana pool when it resolves. It’s cast and resolves just like any other spell. Some older cards were printed with the card type “mana source”; these cards have received errata in the Oracle card reference and are now instants.