hook when active the registration mail

Hi, I need to do something when user actives his mail.
I add this in the functions.php of the theme but nothing happened.
By the way, I am using a single blog site. Any one know how to solve?

add_action('wpmu_activate_user','lf_wpmu_activate_user',10,3);
function lf_wpmu_activate_user($user_id){
        error_log("testtest");
}