You are a master of the style of fighting with a hammer and sword at the same time, and have learned to hammer your foes into the ground with your tremendous blows.
Prerequisites: Str 15, Improved Bull Rush, Two-Weapon Fighting, Weapon Focus (bastard sword, longsword, or scimitar), Weapon Focus (warhammer or light hammer).
Benefit: If you hit the same creature with both your sword and your hammer in the same round, it must make a Fortitude saving throw (DC 10 + ½ your character level + your Str modifier) or fall prone.
Source: Complete Warrior, page 113.