Hooks and single posts problmes

I want to add a stumbleupon button and similar posts html at the end of a post but Not on the main page using hooks. I used the below code but the similar posts and button showed up on the main page. Thanks for the help

function after_post_stuff() {

similar_posts();

echo ‘<p><img src=”https://cdn.stumble-upon.com/images/120x20_su_white.gif&#8221; alt=”” />Stumble It!</p>’;

}
add_action(‘thesis_hook_after_post’, ‘after_post_stuff’);