Missouri Revised Statutes Title XXXV. Civil Procedure and Limitations § 517.132. Written instrument which is basis of claim, counterclaim or cross-claim admissible as evidence, when

If any suit, setoff, counterclaim or cross claim is based upon a written instrument purporting to have been executed by the opposite party, and the same or a verified copy is filed with the court, such instrument shall be admitted in evidence at trial unless the opposing party by responsive pleadings or affidavit shall deny the execution of the instrument, prior to admission of the instrument in evidence.