Reference for Niddah 127:2

רב אמר

but Samuel ruled: No settled period can be established unless the progression is repeated three times.<span class="x" onmousemove="('comment',' Sc. only if in the intercourse given, the discharge had actually appeared on the eighteenth of the fourth month. The appearance on the fifteenth is not counted since it was the first of the series when the process of progression had not yet been apparent (v. infra). ');">²</span>

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