Siman 77

From the Torah, there is no prohibition regarding urine to recite the Shema, to pray, or to bless, except for facing one who is urinating at the time he is urinating, as I wrote in the previous siman. And our sages, of blessed memory, (25a) derived this from verses, as it is written: "And you shall have a place... and you shall go out there," and it is not written: "that you shall cover that which came out of you." And it is written: "And you shall have a spade... and you shall cover your excrement" – here for urine it does not require covering, here for feces it requires covering. And by rabbinic decree they prohibited them even after they fell to the ground, when they have moisture sufficient to moisten. That is to say: when one places something upon the urine – it will be dampened so much that it can moisten another thing when touching it.

A vessel that contains urine—it is forbidden to recite Kiryat Shema facing it, unless one places water into the vessel. And there is no distinction whether the water preceded the urine or the reverse. And we do not say that regarding the second that fell in—the first is nullified bit by bit. Provided that the vessel is not designated for urine. For an earthenware vessel designated for urine—its law is like the urine itself, and it is forbidden to recite and to pray facing it, and it will be explained in siman 87. And it is simple that this refers to where there is no foul odor from the urine, for if there is a foul odor—it is forbidden in any case, even when one placed water into them.

And what is the measure of the water? A revi'it for one instance of urine, and for two instances two revi'it, and so on always. And there is no distinction whether the urine was much or little (Kesef Mishneh and Magen Avraham). And there is one who says that when the urine is less than a revi'it – one does not need a revi'it of water but rather a majority of water (Chacham Tzvi). And some say that even a majority is not required, but rather the same measure as the urine (Shiltei Gibborim). And one may be lenient in a rabbinic prohibition (ibid.). And if the urine is on the ground – one must pour water over the entire area in a place where there is "moisture sufficient to moisten," and one also needs a revi'it.

And know that the Rashba wrote that a revi'it of water is sufficient even for several times. And there is one who says that even according to the opinion of the Rambam, one does not need a revi'it for each time unless he placed water into the urine, and afterward urinated another time and requires other water. But when he has not yet placed water, even if he urinated several times – one revi'it is sufficient. (Mishnah Berurah in the name of Magen Gibborim. And the Gra agreed with the opinion of the Rashba, see there. And one may rely on this in a time of pressing need.)