Source: Complete Mage
You can use your powers of abjuration to interfere with other casters' spells.
Prerequisite: Ability to cast 3rd-level spells.
Benefit: As long as you have an abjuration spell of 3rd level or higher available to cast, you can attempt to interrupt another character's spellcasting with a tiny burst of magic. As an immediate action, you can force any character within 30 feet currently casting a spell to make a Concentration check (DC 15 + the level of the highest-level abjuration spell you have available to cast); if the check fails, the spell's save DC and caster level are reduced by 2 (to a minimum caster level of 1st).
As a secondary benefit, you gain a +1 competence bonus to your caster level when casting abjuration spells.