Stifle Spell

(Player's Handbook II, DnD 3.5)

Abjuration

Level: ; Cleric 4; Sorcerer/Wizard 4
Components: V
Casting Time: 1 immediate action
Range: Close (25 ft. + 5 ft./2 levels)
Target: One creature casting a spell
Duration: Instantaneous
Saving Throw: See text
Spell Resistance: Yes

As the creature casts a spell, you utter a simple magical phrase that momentarily confounds it.
You cast this spell to distract another creature as it attempts to cast a spell.
The target must succeed on a Concentration check (DC equal to 14 + your key ability modifier + the level of the spell being cast) to ignore the distraction, or else it loses the spell.