Reference for Berakhot 49:8

בעו מיניה מרב ששת ריח רע שאין לו עיקר מהו אמר להו אתו חזו הני ציפי דבי רב דהני גנו והני גרסי וה"מ בדברי תורה אבל בק"ש לא ודברי תורה נמי לא אמרן אלא דחבריה אבל דידיה לא:

Rab Sheshet was asked : How is it with a foul odour which has no [material] cause? He replied : Come and behold the mats in the school of Rab, on which some are sleeping while others are studying. But this refers only to words of Torah, not to the reading of the Shema'. And even of words of Torah this is not said except [when the bad odour is caused] by another ; but if he himself is the cause, he must not [even study until the odour has passed off].

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