Manual:$wgRedirectSources

Details
Other wikis which set  to false may redirect users to pages of your wiki, telling your MediaWiki install from what URL they came from. Set this regular expression to restrict origin URLs which will be displayed as "redirected from" links to them.

It might look something like this:

(For instance, this example configuration is pointless because Wikipedia doesn't have hard redirects enabled.)

Leave as false to avoid displaying any incoming redirect markers. This does not affect intra-wiki redirects, which don't change the URL.