Fists of Iron( Oriental Adventures, p. 62)
You have learned the secrets of imbuing your unarmed attacks with extra force.
Improved Unarmed Strike (PH) , Base attack bonus +2,
Declare that you are using this feat before you make your attack roll (thus, a missed attack roll ruins the attempt). You deal an extra 1d4 points of damage when you make a successful unarmed attack. You may use this feat a number of times per day equal to three + your Wisdom modifier.