Familiar Spell( Underdark, p. 25)
You are so well acquainted with the spells you have mastered that you can store the prepared spells in the mind of your familiar.
Spell Mastery (PH) , Ability to acquire a familiar,
You may prepare one spell of any level that you have mastered with the Spell Mastery feat in your familiar's mind instead of your own, treating the extra preparation as if you had one extra spell slot per day, which can be used only for a mastered spell. You can cast this spell normally as long as your familiar is within one square of you. Once cast, the spell is used up, just as if you had held it in your own mind. Your familiar cannot cast this spell itself, even if it is a creature that normally has spellcasting ability.
You may gain this feat multiple times. Each time you take the feat, your familiar's mind can hold one additional prepared mastered spell per day.