Responsa for Niddah 31:17

ושמואל אמר

It was stated: If a woman had a settled period, and when the time of that period arrived she did not make the examination and later she did make one, Rab ruled: If on examination she found that she was unclean she is unclean but if she found that she was clean she remains clean. Samuel, however, ruled, Even if on examination she found herself clean she is deemed unclean, since the guest<span class="x" onmousemove="('comment',' Euphemism, sc. the regular menstrual discharge. ');">¹⁶</span>

Teshuvot Maharam