735 ILCS 5/6-121: Claim of title through common source.

If the plaintiff, or his or her agent or attorney, states under oath that he or she claims title through a common source with the defendant, it is sufficient for the plaintiff to show title from such common source, unless the defendant, or his or her agent or attorney, denies, on oath, that he or she claims title through such source, or swears that he or she claims title through some other source.