halacha - What is the source of the prohibition against "eating or drinking while standing", mentioned in the Kitzur Shulchan Aruch?

The Kitzur Shulchan Aruch in 42:2 states that - "One shouldn't eat while standing and not drink while standing."

Now as I understand, the Kitzur (written by Rabbi Shlomo Ganzfried) is a summary of the Shulchan Aruch, which in turn is largely based on the Beit Yosef, which is a commentary on the Arba'ah Turim.

Is this prohibition mentioned in the Shulchan Aruch, Beit Yosef, Tur or any other work? Or did Rabbi Ganzfried add it based on his own knowledge?

This is a related question which was asked on Mi Yodeya, however it does not mention the verse in the Kitzur. Forbidden to eat while standing up?

Answer

The source for this is the Masechet Derech Eretz; one of the minor tractates (4:1):

מי שהוא תלמיד חכם, לא יאכל מעומד, ולא ישתה מעומד, ולא יקנח את הקערה, ולא ילקק באצבעותיו, ולא יגהק בפני חבירו, מיעוט שיחה, מיעוט שחוק, מיעוט שינה, מיעוט תענוג, מיעוט הן הן, מיעוט לאו לאו

It gives a list of personal habits that Torah scholars should adopt including not eating and drinking while standing.

This is mentioned in many post-Talmudic sources and was not made up by R. Ganzfreid. Such as the Rambam (De'ot 4:3) the Hagahot of Rama to the Mordechai in B'rachot (ch. Keitzad Mevarchim: remez 145).

It should be noted that Rambam writes merely that a person should do so; not that it is prohibited to do otherwise.

It should be noted that much of Hilchot De'ot of the Rambam is quoted almost verbatim by the Kitzur.

This advice is notably absent from the Shulchan Aruch.