What is the source of the prohibition of not preparing on Shabbat for after Shabbat?

Question

Preparing on Shabbat for after Shabbat is not one of the 39 melachot (Biblical categories of "work").

I gather that many prohibitions are gleaned from Yeshayahu (Isaiah) 58:13. However, the only thing I could see in this verse is the concept of how to honor Shabbat with one of the ways being the phrase Asot Cheftzecha. There does not appear to be a specific direct prohibition implied, here. Was this prohibition placed as a syag - a "fence" to prevent some other melacha from happening?

Answer 1

Shabbos 113a says that one may not prepare the bedspreads from Shabbos to Motzei Shabbos. מציעין את המיטות מלילי שבת לשבת, אבל לא משבת למוצאי שבת

Mishnas Avraham in the Semag - volume 2 - 71 quoting the Raavad regarding cleaning dishes from Shabbos for the weekday says it is forbidden as it is exerting oneself for the weekday.

I do not think this is a Seyog to prevent other Melacha from happening. It sounds to me more like a lack of Kavod Shabbos.