Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 217, § 5A

Each register, before entering upon the performance of his official duties, in addition to the oaths prescribed by the constitution, shall take and subscribe an oath that he will faithfully discharge said duties and that he will not during his continuance in office, directly or indirectly, be interested in, or benefited by, the fees or emoluments which may arise in any suit or matter pending in either of the courts of which he is register. Such oaths shall be filed in the registry of probate of the county for which he is elected.