Crystal Slipper

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2019-10-04
  Official Ruling
  If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won't be able to attack. This means that you can't use one Crystal Slipper to allow two new creatures to attack in the same combat.

Rulings via Scryfall