Mage-Ring Bully

• WOTC Official Reminder Textofficial rules
  Prowess — Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.

• WWizards of the Coast2015-06-22
  Official Ruling
  If, during its controller's declare attackers step, Mage-Ring Bully is tapped or is affected by a spell or ability that says it can't attack, then Mage-Ring Bully doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost. If they don't, Mage-Ring Bully doesn't have to attack.
• WWizards of the Coast2015-06-22
  Official Ruling
  Once it triggers, prowess isn't connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
• WWizards of the Coast2015-06-22
  Official Ruling
  Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
• WWizards of the Coast2015-06-22
  Official Ruling
  Any spell you cast that doesn't have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won't cause prowess to trigger. Playing a land also won't cause prowess to trigger.

Rulings via Scryfall