Md. Code, Courts and Judicial Proceedings § 9-116

A party to a proceeding by or against a personal representative, heir, devisee, distributee, or legatee as such, in which a judgment or decree may be rendered for or against them, or by or against an incompetent person, may not testify concerning any transaction with or statement made by the dead or incompetent person, personally or through an agent since dead, unless called to testify by the opposite party, or unless the testimony of the dead or incompetent person has been given already in evidence in the same proceeding concerning the same transaction or statement.