htaccess redirect domain to a page on another domain

RewriteCond %{HTTP_HOST} ^(www\.)?domain1.com$ [NC]
RewriteRule ^oldpagename$ http://externalsite/page [R=301,L]

from: http://stackoverflow.com/questions/11540138/htaccess-rewrite-rule-to-redirect-specific-page-on-specific-domain-to-another-ur