Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 3, Opening Comments
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בּוֹ יְבֹאַר שֶׁאִסּוּר רְכִילוּת הוּא בֵּין בְּפָנָיו וּבֵין שֶׁלֹא בְּפָנָיו, וּבוֹ ד' סְעִיפִים.
Chofetz Chaim, Silverstein translation
In this principle it will be explained that the issur of rechiluth obtains both in his [the object's] presence and not in his presence. It contains four sections.