binded textBox update

Question

I am Using a Textbox and a button for submiting textbox text , the button should be unavailable if there is No text , but , the textbox "Updates" her Binded property (whitch used by buttun can execute comman ) after it lost focus , and I want the text box to update after each keystroke ,
I know there is a property that can solve my problem I cant remmember how it's called

thank you ...