post_prev & post_next within same category

Add this in your functions file and it links all single posts within the same category

add_action('loop_end', 'wpsites_nav_links', 25 );
function wpsites_nav_links() {
if( !is_single() ) 
return;

previous_post_link('<span class="single-post-nav previous-post-link">%link</span>', '&lt;&lt; Previous Post in Category', TRUE);
next_post_link('<span class="single-post-nav next-post-link">%link</span>', 'Next Post in Category &gt;&gt;', TRUE);
}