How to get WordPress post featured image URL

I am using this function to get the featured images:

<a href="#" rel="prettyPhoto">
    <?php the_post_thumbnail('thumbnail'); ?>
</a>

Now I want to get the full featured image on click on the anchor tag for which I need a featured image URL in

<a href="here" rel="prettyPhoto">

How can I fix this?

Check the code below and let me know if it works for you.

<?php if (has_post_thumbnail( $post->ID ) ): ?>
  <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
  <div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">

  </div>
<?php endif; ?>

If you want JUST the source, and not an array with other information:

<?php $url = wp_get_attachment_url( get_post_thumbnail_id($post->ID), 'thumbnail' ); ?>
<img src="<?php echo $url ?>" />

// Try it inside loop.  
<?php
$feat_image = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
echo $feat_image;
?>

Easy way!

 <?php 
     wp_get_attachment_url(get_post_thumbnail_id(get_the_ID()))
 ?>

This perfectly worked for me:

<?php echo get_the_post_thumbnail_url($post_id, 'thumbnail'); ?>