25 ILCS 5/2

Every officer of each house of the General Assembly shall, before entering upon the duties of his office, take and subscribe the following oath, which shall be filed with the Secretary of State:

"I do solemnly swear (or affirm, as the case may be,) that I will support the constitution of the United States, and the constitution of the state of Illinois, and that I will faithfully discharge the duties of the office of .... according to the best of my ability."