[SalesForce] get the Aura:id from event

How can i get the aura:id dynamically from event attribute, the below example i was trying to get the aura:id to get the value, else you can think as aura:id might be some field APIname so i can pass this field name and value to controller to store the value dynamically.

code
component:

<aura:component controller = "LC_AutoSave" implements="force:appHostable" >
    <form>
        <ui:inputtext  value = ""  aura:id ="testtext"  blur = "{!c.autoSave}"/>
        <ui:inputtext value = "" aura:id ="testtext1"  blur = "{!c.autoSave}"/>
        <ui:inputtext value = "" aura:id ="testtext2"  blur = "{!c.autoSave}"/>

   </form>
</aura:component>

Controller

({
    autoSave : function(component, event, helper) {
        var a = event.getSource();
        console.log('Test'+a.get("v.aura:Id"));
        helper.autoSavehelper(component,event);
    }
})