Reference for Ketubot 210:9

כי הא דשמואל הוה עבר במברא אתא ההוא גברא יהיב ליה ידיה אמר ליה מאי עבידתיך אמר ליה דינא אית לי א"ל פסילנא לך לדינא

Like the case of Shmuel who was once crossing [a river] on a bridge. A man came up and offered him his hand. [Shmuel] asked him, “What is your business here?” He said to him, “I have a court case.” [Shmuel] replied, “I am disqualified from acting as your judge.”

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