Mishnah 5

הָיָה עוֹמֵד בַּתְּפִלָּה, וְנִזְכַּר שֶׁהוּא בַּעַל קֶרִי - לֹא יַפְסִיק, אֶלָּא יְקַצֵּר. יָרַד לִטְבֹּל: אִם יָכוֹל לַעֲלוֹת וּלְהִתְכַּסּוֹת וְלִקְרוֹת עַד שֶׁלֹּא תָּנֵץ הַחַמָּה - יַעֲלֶה וְיִתְכַּסֶּה, וְיִקְרָא. וְאִם לָאו - יִתְכַּסֶּה בַּמַּיִם, וְיִקְרָא. אֲבָל לֹא יִתְכַּסֶּה, לֹא בְּמַיִם הָרָעִים, וְלֹא בְּמֵי הַמִּשְׁרָה, עַד שֶׁיַּטִּיל לְתוֹכָן מַיִם. וְכַמָּה יַרְחִיק מֵהֶם וּמִן הַצּוֹאָה? אַרְבַּע אַמּוֹת.

If one was standing in prayer and remembers that he is a baal keri, he does not stop praying, rather he should abbreviate the prayer. If he went down to immerse in a mikvah, if he is able to come up, cover himself and recite the Shema before sunrise – he should rise, cover himself and recite the Shema. But if he is not able to do so – he should cover himself in the water and recite the Shema. But he may not cover himself, not in foul waters, and not in water in-which flax was soaked, (one may not recite the Shema in front of urine); until he pours water into it. And how much must he distance himself from it [urine] and excrement? Four amot – forearms (i.e.6 feet).