Talmud, Shabbat 124b
Shabbat 124b:3
Discusses the Talmudic basis for muktzeh, including the prohibition against moving certain objects on Shabbat and the reasoning behind the categories.
וְאַף רַב סָבַר לַהּ לְהָא דְּרָבָא. דְּאָמַר רַב: מָר שֶׁלֹּא יִגָּנֵב — זֶהוּ טִלְטוּל שֶׁלֹּא לְצוֹרֶךְ, וְאָסוּר. טַעְמָא שֶׁלֹּא יִגָּנֵב, אֲבָל לְצוֹרֶךְ גּוּפוֹ וּלְצוֹרֶךְ מְקוֹמוֹ — מוּתָּר. אִינִי?! וְהָא רַב כָּהֲנָא אִיקְּלַע לְבֵי רַב, וַאֲמַר: אַיְיתוֹ לֵיהּ שׁוּתָא לְכָהֲנָא לִיתִּיב עֲלַהּ. לָאו לְמֵימְרָא דְּדָבָר שֶׁמְּלַאכְתּוֹ לְאִיסּוּר, לְצוֹרֶךְ גּוּפוֹ — אִין, לְצוֹרֶךְ מְקוֹמוֹ — לָא!
The Gemara adds: And even Rav holds in accordance with this halakha of Rava, as Rav said: Moving a hoe so that it will not be stolen; that is an example of moving an object not for a specific purpose, and it is prohibited. The Gemara infers: The reason that it is prohibited is that it is moved so it will not be stolen; however, for the purpose of utilizing the object itself and for the purpose of utilizing its place, it is permitted. The Gemara asks: Is that so? Didn’t Rav Kahana happen to come to the house of Rav, and he said: Bring a net for Kahana so that he may sit on it? Is that not to say that with regard to an object whose primary function is prohibited, for the purpose of utilizing the object itself, yes, it is permitted to move it; and for the purpose of utilizing its place, no, it is prohibited?