Mana source is an obsolete card type that was only found on Culling the Weak and Dark Ritual, as well as the non-tournament-legal Blacker Lotus and Jack-in-the-Mox, beginning with the printing in Mirage until Urza's Saga. All mana sources would have been interrupts under pre-Mirage rules, and are now instants. Abilities that were played as mana sources were previously played as interrupts, and are now mana abilities.
From the glossary of the Comprehensive Rules (Archenemy: Nicol Bolas (June 5, 2017))
- Mana Source (Obsolete)
- An obsolete card type. All cards printed with this card type are now instants. All abilities that, as printed, said a player could “play as a mana source” are now mana abilities. All relevant cards have been given errata in the Oracle card reference.