Responsa for Niddah 31:5

ואיבעית אימא

R. Nahman: [Is the examination at] regular menstrual periods Pentateuchal<span class="x" onmousemove="('comment',' So that if a woman failed to make the examination at the proper time she is deemed to be unclean (on the ground that the discharge had appeared at its usual time) even though she observed no blood when she examined herself some time later (since it might have dropped on the ground and been lost). ');">⁵</span>

Teshuvot Maharam