Glibness

Level: 8

Casting Time: 1 action

Range: Self

Duration: 1 hour

Until the spell ends, when you make a Charisma check, you can replace the number you roll with a 15. Additionally, no matter what you say, magic that would determine if you are telling the truth indicates that you are being truthful.

Source: PHB, page 245. Available in the SRD.