Reference for Berakhot 49:16

הא כיצד כאן בגדולים כאן בקטנים אלמא קטנים לא אסרה תורה אלא כנגד עמוד בלבד הא נפול לארעא שרי ורבנן הוא דגזרו בהו וכי גזרו בהו רבנן בודאן אבל בספקן לא גזור

how is this? The latter verse refers to the major natural function, the former to the minor. Infer, then, that the Torah forbids [the Shema' to be read] near to urine only when it is by fecal matter ; but if it has fallen upon the ground, it is permitted [by the Torah], but the Rabbis have decreed against it. Have the Rabbis decreed against it ? When it is certain [that urine is there] ; but if there is a doubt, they do not decree against it.

Jastrow