Redirect iRule

If you are using 11.4.0 or higher, you do not need to use an iRule. Instead, you can use a Local Traffic Policy:

https://support.f5.com/kb/en-us/solutions/public/15000/000/sol15085.html

If you are using a version before 11.4.0, you can use HTTP Classes:

https://support.f5.com/kb/en-us/solutions/public/7000/200/sol7280.html

Using one of these options is generally better than using an iRule. However, for completeness, here is an iRule to accomplish what you desire:

when HTTP_REQUEST {
    if { [string tolower [HTTP::host]] eq "something.com" } {
        HTTP::redirect "http://anything.com[HTTP::uri]"
    }
}

I assume here that you want to redirect any arbitrary URI path from source to destination, keeping the URI path intact on the redirect.