Mage Armor

Level: 1

Casting Time: 1 action

Range: Touch

Duration: 8 hours

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.

Source: PHB, page 256. Available in the SRD.