X11 forwarding after sudo/su

$ ssh -X userA@host
password: <userA password>

$ xauth list ${DISPLAY#localhost}
host/unix:15  MIT-MAGIC-COOKIE-1  e7956ac62f6f5c3448cef9b14783d4ff


$ sudo su userB
password: <userB password>

$ xauth add  host/unix:15  MIT-MAGIC-COOKIE-1  e7956ac62f6f5c3448cef9b14783d4ff

$ #done. :) ## :)
$ #To verify
$ xterm