En fait, une attestation sur l'honneur est une "lettre" dans laquelle une personne certifie quelque chose et de ce fait s'engage. Attestation sur l'honneur is the "name" of this letter. Maurits is right. But you can also just write a letter with this tittle "Attestation sur l'honneur...
This document is commonplace in France, and it is possible to write your own Declaration or Attestation sur l’Honneur. When might I need an Attestation sur L’Honneur? There are many situations where you might be asked to write an attestation, often for tax, social security...