functions.php

<?php
/* Redirect to post if search results only returns one post.
   http://blog.wpjam.com/m/redirect-to-post-if-search-results-only-returns-one-post/
   ------------------------------------------------------------------------------ */
add_action('template_redirect', 'search_redirect_post');

function search_redirect_post() {
    if (is_search()) {
        global $wp_query;
        if ($wp_query->post_count == 1) {
            wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
        }
    }
}