Mo. Rev. Stat. § 84.040
The said commissioners shall be citizens of the state of Missouri, and shall have been residents of the cities for a period of four years next preceding their appointment; they shall, except as specified in sections 84.030 and 84.080, hold their offices for four years, and until their respective successors shall have been appointed and qualified, and receive each a salary of one thousand dollars per annum, payable monthly; before entering upon the duties of their said offices, the said commissioners and the said mayor shall take and subscribe before a circuit or associate circuit judge of the circuit court of judicial circuit in which said cities shall be located, or the clerk thereof, the oath or affirmation prescribed by the Constitution of the state of Missouri, and shall also take and subscribe before the same judge or clerk the further oath or affidavit that in any and every appointment or removal to be made by them to or from the police force created and to be organized by them under sections 84.010 to 84.340, they will in no case and under no pretext appoint or remove any policeman or officer of police, or other person under them, on account of the political opinions of such police officer or other person, or for any other cause or reason than the fitness or unfitness of such a person, in the best judgment of such commissioners, for the place for which he shall be appointed, or from the place from which he shall be removed. The said oaths or affirmations shall be recorded and preserved among the records of the said circuit court.
§ 84.040, RSMo