Missouri Constitution
Article III - Legislative Department
Section 15 Oath of office of members of assembly—administration—ef fect of refusal to take oath and conviction of violation.

Every senator or representative elect, before entering upon the duties of his office, shall take and subscribe the following oath or affirmation: “I do solemnly swear, or affirm, that I will support the Constitution of the United States and of the state of Missouri, and faithfully perform the duties of my office, and that I will not knowingly receive, directly or indirectly, any money or other valuable thing for the performance or nonperformance of any act or duty pertaining to my office, other than the compensation allowed by law.” The oath shall be administered in the halls of the respective houses to the members thereof, by a judge of the supreme court or a circuit court, or after the organization by the presiding officer of either house, and shall be filed in the office of the secretary of state. Any senator or representative refusing to take said oath or affirmation shall be deemed to have vacated his office, and any member convicted of having violated his oath or affirmation shall be deemed guilty of perjury, and be forever disqualified from holding any office of trust or profit in this state.

Source: Const. of 1875, Art. III, Sec. 15.