אורח חיים ע״זOrach Chayim 77

Se’if 1א׳

אָסוּר לִקְרוֹת כְּנֶגֶד מֵי רַגְלַיִם, עַד שֶׁיָּטִיל לְתוֹכָן רְבִיעִית מַיִם, וְאָז מֻתָּר. וְלָא שְׁנָא עַל גַּבֵּי קַרְקַע, לָא שְׁנָא בִּכְלִי, וּבִלְבַד שֶׁלֹּא יְהֵא עָבִיט הַמְיֻחָד לָהֶם. וְדִין עָבִיט ע"ל סִימָן פ"ז לָא שְׁנָא הָיוּ הֵם בַּכְּלִי תְּחִלָּה, וְנִתַּן עֲלֵיהֶם מַיִם, לָא שְׁנָא הַמַּיִם בַּכְּלִי תְּחִלָּה.

Not to Recite [the Recitation of the Shema] Opposite Urine - Containing 2 Se'ifim It is forbidden to recite opposite urine until one has poured a revi-it of water into it, and then it is permitted. And it makes no difference whether it is on the surface of the ground or in a vessel - as long as it is not in a special chamber pot that's designated for it [And for the law of the chamber pot see below Siman 87]. It makes no difference whether it was in a vessel first and water was poured into it or the water is in the vessel first.

Se’if 2ב׳

רְבִיעִית שֶׁאָמְרוּ, לְמֵי רַגְלַיִם שֵׁל פַּעַם אַחַת. וּלְשֶׁל שְׁנֵי פְּעָמִים שְׁנֵי רְבִיעִיּוֹת, וּלְשֶׁל שָׁלֹשׁ שְׁלֹשָׁה: הַגָּה: וְכֵן לְעוֹלָם:

A revi-it [of water], that they said [is required to be poured onto urine in order to render it permissible to pray in front of] is for one urination, and for two times (ie. two urinations), two revi-i-ot [are required], and for three [urinations], three [revi'ot are required]Rem"a: And so on and so on.