You can use is_single()
in a conditional statement.
<?php if( is_single() ){ ?>
<ul>
<li class="Source"><a>source:</a></li>
<ul class="Source-Link">
<li><a href="https://wordpress.stackexchange.com/questions/121060/<?php echo get_post_meta($post->ID,"source-link', true); ?>" target="_blank"><?php echo get_post_meta($post->ID, 'source', true); ?></a></li>
</ul>
</li>
</ul>
<?php }?>
Be aware though that this will appear on any single page, meaning a single post page and a single page.