Nedarim 39A

גמ׳ במאי עסקינן? אי בשנכסי מבקר אסורין על חולה ⁠— אפילו יושב נמי? אי בשנכסי חולה אסורין על המבקר ⁠— אפילו עומד נמי לא! אמר שמואל: לעולם בשנכסי מבקר אסורין על החולה, ובמקום שנוטלין שכר על הישיבה, ואין נוטלין שכר על העמידה.

Talmud: With what are we dealing? If it is a case where the property of the visitor is forbidden to the ill person, even if he is sitting, this should also be permitted. If it is a case where the property of the ill person is forbidden to the visitor, even if he is standing, it should also not be permitted, as one derives benefit from entering the house. Shmuel said: Actually, it is a case where the property of the visitor is forbidden to the ill person, and it is in a place where one takes payment for visiting and sitting with an ill person and one does not take payment for visiting and standing with an ill person. Therefore, by sitting with the ill person the visitor provides him forbidden benefit by sparing him the expense of hiring another person to sit with him.

מאי פסקא? הא קא משמע לן דאף במקום שנוטלין שכר, על הישיבה ⁠— בעי למשקל, על העמידה ⁠— לא בעי למשקל. ואיבעית אימא: כדרבי שמעון בן אליקים, גזירה שמא ישהא בעמידה. הכא נמי גזירה שמא ישהא בישיבה.

Why was this distinction stated without qualification? There is no apparent fundamental difference between sitting and standing when visiting the ill. The Talmud answers: It teaches us this: Even in a place where one takes payment for visiting the ill, for sitting, one ought to take payment, but for standing, one ought not to take payment. And if you wish, say instead that the distinction can be explained in accordance with the statement that R' Shimon ben Elyakim said elsewhere (42a), that one who is prohibited to derive benefit from another due to a rabbinic decree may not enter a field that is owned by the latter, lest he remain standing there longer than permitted. Here too, sitting is prohibited due to a rabbinic decree, lest he remain sitting there longer than is necessary to perform the mitzva of visiting the ill.