Commentary for Berakhot 117:5

א"ר קטינא אובא טמיא כדיב הוא ומיליה כדיבין אי הכי גוהא גוהא מיבעי ליה ולא היא גוהא גוהא עביד והאי דלא אודי ליה כי היכי דלא ליטעי כולי עלמא אבתריה

Rab Kattina said, "The necromancer lies and his words are false; for in that case, there should be one rumbling noise followed by another!" But it is not so ; there is really one rumbling noise followed by another, and the reason that Kattina did not acknowledge the necromancer's statement was that people should not be led astray after him.

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot