Mass. Gen. Laws ch. 40, § 46

Any person authorized to approve a claim against a town for labor, materials or service may, before approval thereof, require the claimant to certify under oath that all the articles for which claim is made have been furnished, or that all the labor or service has been performed, and that no commission, discount, bonus, reward or present of any kind has been received by, or promised to, or is expected by, any person on account of the same.