Reference for Sotah 4:21
סתירה נמי איתקש לטומאה דכתיב ונסתרה והיא נטמאה ההוא לכמה שיעור סתירה כדי טומאה הוא דאתא
What is the reason of R. Jose son of R. Judah? — Scripture states 'against her', i.e., 'against her' [in the matter of misconduct] but not in the matter of seclusion. Perhaps, however, 'against her' means: and not in the matter of warning? — Warning is compared to 'defilement', for it is written, and he be jealous of his wife and she be defiled. But seclusion is also compared to 'defilement', for it is written, and he kept close and she be defiled? — That refers to a length of time sufficient for 'defilement' to have occurred.<span class="x" onmousemove="('comment',' So that if the time of seclusion was insufficient, she is not required to drink the water. ');"><sup>19</sup></span>