Instead of adding a function to your functions.php file I am inclined to agree with Gangleri and suggest you add it to the template.
Just below where your single.php (for permalink pages) has
<?php the_content(); ?> put what you want to display.
<div class="postfoot">This post was by .....</div>
If you don't have a single.php file in your theme, add it below
<?php the_content(); ?> like this to limit it to permalink pages only:
<?php if(is_single()): ?><div class="postfoot">This post was by .....</div><?php endif; ?>
If you want it to display the name dynamically by the author of the post you use the author template tags. Like:
<div class="postfoot">This post was by <?php the_author(); ?> is <?php the_author_description(); ?></div>
I like using the author description cause it allows you to change the description in the admin interface in the profile.