Bava Batra 160a בבא בתרא ק״ס א

MISHNA: In an ordinary document, its witnesses are to sign inside it, i.e., on the written side of the paper. In a folded and tied document, its witnesses are to sign on the back of it.

With regard to an ordinary document whose witnesses wrote their signatures on the back of it, and a tied document whose witnesses wrote their signatures inside of it, both of these are not valid. Rabbi Ḥanina ben Gamliel says: A tied document whose witnesses wrote their signatures inside of it is valid, because one can transform it into an ordinary document by untying it. Rabban Shimon ben Gamliel says: Everything is in accordance with regional custom.

An ordinary document is rendered valid by its having at least two witnesses, and a tied document is rendered valid by its having at least three witnesses. With regard to an ordinary document in which a single witness wrote his signature, and a tied document in which only two witnesses wrote their signatures, they are both not valid.

GEMARA: The Gemara asks: From where are these matters derived? What biblical basis is there for the existence of these two types of documents? Rabbi Ḥanina says: As the verse states: “They shall buy fields for money, and subscribe the deeds, and seal them, and call witnesses” (Jeremiah 32:44). When the verse states: “They shall buy fields for money, and subscribe the deeds,”