Manual:$wgRedirectSources

Details
Other wikis which set $wgDisableHardRedirects 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.