Overwhelm (MM2) from Modern Masters 2015, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Wayne Reynolds
• Card Text:
  Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for 1 or one mana of that creature's color.)
  Creatures you control get +3/+3 until end of turn.