Missing argument 2 for add_ation(),

Hello
I am trying to show text that was placed in the “headway_before_entry_content” hook, but ONLY on single posts and not the front page or blog index. this is the code and text i used inside “headway_before_entry_content” hook

<?php
if(is_single() ) {
        add_action('headway_before_entry_content');
    }
?>
<h2 class="Lorem-Ipsum">Lorem Ipsum
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi sodales, nulla eu gravida vulputate, sapien est viverra neque, ac accumsan lorem tellus porta magna. Sed eget vehicula turpis . </h2>

the problem is on single posts im getting an error message
“Warning: Missing argument 2 for add_action()” what could it be?

http://ww.wp.xz.cn/plugins/thesis-openhook/