Function to get thumbnail img source

wp_get_attachment_url( get_post_thumbnail_id($post->ID) );

all together: 

    <div class="entry-thumbnail">
       <a rel="lightbox" href="https://wordpress.stackexchange.com/questions/107218/<?php echo wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); ?>"><?php echo get_the_post_thumbnail( $post->ID ); ?></a>
    </div>

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