Reference for Berakhot 113:20

תני תנא קמיה דרב ששת הרואה נחש בחלום פרנסתו מזומנת לו נשכו נכפלה לו הרגו אבדה פרנסתו אמר ליה רב ששת כל שכן שנכפלה פרנסתו ולא היא רב ששת הוא דחזא חויא בחלמיה וקטליה

A Tanna taught in the presence of Rab Sheshet ; Who dreams of a serpent, his sustenance will be provided him ; if it bit him, his sustenance will be doubled for him ; if he killed it, his sustenance is lost. Rab Sheshet said to him : [When he dreams of killing the serpent], how much more likely is it that his sustenance will be doubled ! But it is not so ; and it was Rab Sheshet who dreamt that he saw a serpent and killed it.

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