Reference for Berakhot 21:18

וכי תימא בית הלל נמי אית להו מטין ה"מ דמטה ואתא מעיקרא אבל הכא כיון דעד השתא הוית זקוף והשתא מוטה אמרי ש"מ כב"ש סבירא להו שמא יראו התלמידים ויקבעו הלכה לדורות.

Shouldest thou say, "Bet Hillel also hold [that the Shema' may be read] in a reclining position!" that is so, but only when one was reclining from the first ; but in thy case, since hitherto thou has been sitting up and now thou reclinest, they will say, "We have to infer that [our teachers] agree with the opinion of Bet Shammai," and there is a fear, that the disciples seeing this, will fix the Halakah accordingly for future generations.

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