One is forbidden to recite the Shema while in the presence of excrement, for in the Torah it is written, “Cover up your excrement, because the Lord your G-d moves about in your camp” (Deuteronomy 23:14-15). Therefore, if there is excrement in a room and a person wants to recite the Shema, he must either cover it or remove it. If there is only a very small amount of excrement it is sufficient to spit upon it. The sage Rava adds the proviso that the spit must be thick (a large quantity of spit). If the excrement is within a depression, he may place his shoe over the hole and recite the Shema. One of the sages asked what the rule is if the excrement is stuck to his shoe. Is one permitted to recite the Shema, or is one forbidden? What are the issues under debate? Does his shoe serve as a cover for the excrement, as required by the Torah’s demand “cover up your excrement,” or since the excrement is stuck to the shoe, is it not considered a separate cover? (The Talmudic sages did not reach a decision on this question.)
(Babylonian Talmud, Tractate Berachot 25b)