All you have to do is include the following inside your main WP loop, underneath your featured thumbnail…

<?php 
$thumb_id = get_post_thumbnail_id($post->ID);
$thumb_title = get_the_title($thumb_id); 
?>

<?php if(!empty(get_post($thumb_id)->post_excerpt)) : ?>
<figcaption class="wp-caption-text">
<?php echo get_post($thumb_id)->post_excerpt; ?>

<?php 
$source_url = get_post_meta($thumb_id, 'source_url', true);
$parts = parse_url($source_url);
if(!empty($source_url))
echo ' (via: <a href="' . $source_url . '" target="_blank">' . $parts['host'] . '</a>)';
?>
</figcaption>
<?php endif; ?>

If you aren’t using HTML5, then make sure to use something different than the <figcaption> tag, like a <div> or <span>.