Momentum is a uncommon Enchantment — Aura card from Urza's Destiny (UDS), collector number 113. Current market price: $0.22 (foil $4.23), TCGplayer market prices via Scryfall.
Enchant creature At the beginning of your upkeep, you may put a growth counter on this Aura. Enchanted creature gets +1/+1 for each growth counter on this Aura.