Reference for Bava Batra 80:1
דלא כתיב בה אנן ידעינן ביה באונסא דפלניא לאו מודעא היא
that does not contain the words 'we, [the undersigned] are cognisant that so-and-so is acting under duress', is no moda'ah. Of what kind of moda'ah are we speaking? If of one relating to a get [bill of divorce] or a gift. [why should the witnesses have to make this declaration, seeing that] it [only states something which] is more or less self-evident?<span class="x" onmousemove="('comment',' In the case of a get or a gift, there is no motive for a man to say that he is acting under constraint unless this is actually the case; hence there is no reason why the witnesses should have independent knowledge of the fact. In the case of a sale, however, it may happen that a man sells something in order to raise money, but with the idea of buying it back as soon as possible, and he may therefore be tempted to issue a moda'ah falsely in order to facilitate this. ');"><sup>1</sup></span>