Mich. Comp. Laws § 600.2141

Sec. 2141. In any suit or proceeding hereafter instituted in any of the courts of this state, wherein it shall become material or necessary to prove the copartnership of any firm or association the plaintiffs may cause to be served upon the defendant, with a copy of the complaint filed in the cause, or with the process by which suit is commenced, an affidavit stating that the plaintiffs were the persons comprising such partnership at the time the contract in question was made, or the cause of action accrued; and such affidavit shall be prima facie evidence of such existence of such partnership or association, unless the defendant shall file with his plea an affidavit denying the existence of such partnership or association.