סימן פח סעיף יח

מֻתָּר לִתֵּן כְּלִי תַּחַת הַדֶּלֶף בַּשַׁבָּת. וְאִם נִתְמַלֵּא, שׁוֹפְכוֹ וּמַחְזִירוֹ לִמְקוֹמוֹ, וְהוּא שֶׁיְהֵא הַדֶּלֶף רָאוּי לִרְחִיצַה. אֲבָל אִם הַמַּיִם מְאוּסִים, אָסוּר לִתֵּן שָׁם כְּלִי, מִשּׁוּם דְּאֵין עוֹשִׂין גְּרָף שֶׁל רְעִי לְכַתְּחִלָּה. וְאִם עָבַר וּנְתָנוֹ וְהוּא בְּמָקוֹם דְּמָאִיס עָלָיו, מֻתָּר לְהוֹצִיאוֹ (דִין מֻקְּצָה בִּמְקוֹם הֶפְסֵד גָּדוֹל, עַיֵן לְעֵיל סִימָן פ"ה סָעִיף ד'. וְדִינֵי מֻקְצֶה בְּיוֹם טוֹב וְדִין מֻקְצֶה מֵחֲמַת מִצְוָה, יְבֹאֲרוּ אִם יִרְצֶה הַשֵׁם בִמְקוֹמָם).

You may place a vessel under a leak on Shabbos. When it fills, you may empty it and put it back in its place, provided the dripping [water] is fit for washing. If, however, the water is dirty, you are forbidden to place a vessel there, because it is forbidden to create a "new bedpan."⁶⁵Explanation: making a vessel that had other uses into a bedpan “destroys” the potential of the other uses. The Rabanan found this comparable to “destroying,” (סְתִּירָה) —a melachah prohibited on Shabbos.
Note: This ruling of the author is based on Shulchan Aruch and Rambam. Tur, however, is more lenient and rules that you may place the vessel there even if the water is not fit for anything. Bi’ur Halachah (338) points to this ruling as the source for our washing our hands in the morning into a vessel (“negl-vasser“) though the water is totally unfit after washing (v. Chapter, paragraph 2). The same applies to “mayim acharonim“ —the washing done before Birkas Hamazon. If you transgressed and placed [the vessel there] and it is in a place where it is repugnant to you, you may remove [the water]. (For the laws of muktzeh involving great monetary loss—see above Chapter 85, paragraph 4. The laws of muktzeh on Yom Tov and the laws of muktzeh due to a mitzvah, will be explained, God willing, in their respective places.)

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)