Mishnah Ketuboth

Chapter 2, Mishnah 3

If one witness says, "this is my handwriting and that is the handwriting of my fellow", and another witness says, "this is my handwriting and that is the handwriting of my fellow", they are believed. if one says, "this is my handwriting" and the other says, "this is my handwriting", they must join to themselves another person. this is the view of Rabbi. but the sages say: "they need not join to themselves another person, but a person is believed to say, this is my handwriting."