yytz: It is a Gemara in Horiyos. I do not know if it is brought down in the Mechaber but I do know that the Aruch Hashulchan mentions it and I think the Mishnah B’rurah also brings down the list of 10 things that are Kasheh L’shich’chah. It’s not an Issur per se, but good advice if you don’t want spiritual/Kabbalistic impediments towards remembering your learning.
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