Zone of Truth

This spell prevents creatures within the area of effect (or those who enter it) from speaking any deliberate and knowing lies. Creatures are allowed a saving throw to avoid the effects; those who fail the save are affected fully. Affected characters are aware of this enchantment; therefore, they may avoid answering questions to which they would normally respond with a lie or they may be evasive as long as they remain within the boundaries of the truth. When a character leaves the area, he is free to speak as he chooses.
The spell affects a square whose sides are five feet long per level of the caster; thus, a 4th-level priest could affect a 20 foot by 20 foot square.

[Tome of Magic]

Sphere: Wards
(Enchantment/Charm)
Level: 2

Range: 30 yards
Components: V, S, M
Duration: 1 round/level
Casting Time: 2
Area of Effect: 5-foot square/level
Saving Throw: Negates

Material Components:
the priest's holy symbol and a phony emerald, ruby, or diamond