Reference for Ketubot 54:12

א"ל זיל אייתי סהדי דאחוה את א"ל אית לי סהדי ומסתפינו מיניה דגברא אלמא הוא א"ל לדידיה זיל אייתינהו את דלאו אחוך הוא א"ל דינא הכי המוציא מחבירו עליו הראיה א"ל הכי דאיננא לך ולכולהו אלמי חברך השתא נמי אתו ומשקרי תרתי לא עבדי

[Then] he said to him: I Go and bring witnesses that you are his brother. He answered him: I have witnesses, but they are afraid of him, because he is a powerful man. He [then] said to the other man: Go you and bring witnesses that he is not your brother. He answered him: Is this the law? [Surely] he who wishes to extract money from his fellow, must provide the evidence. He said to him: This is how I rule for you and all who are powerful like you! But they might still come and lie? Two things they will not do.

Jastrow