Wurm's Tooth

• WWizards of the Coast2009-10-01
  Official Ruling
  The ability triggers whenever any player, not just you, casts a green spell.
• WWizards of the Coast2009-10-01
  Official Ruling
  If a player casts a green spell, Wurm’s Tooth’s ability triggers and is put on the stack on top of that spell. Wurm’s Tooth’s ability will resolve (causing you to gain 1 life) before the spell does.

Rulings via Scryfall