Manusmriti

siva.sh/manusmriti/3/46

ऋतुः स्वाभाविकः स्त्रीणां रात्रयः षोडश स्मृताः । चतुर्भिरितरैः सार्धमहोभिः सद्विगर्हितैः ॥ ४६ ॥

sanskrit

Sixteen (days and) nights (in each month), including four days which differ from the rest and are censured by the virtuous, (are called) the natural season of women.

english translation

RtuH svAbhAvikaH strINAM rAtrayaH SoDaza smRtAH | caturbhiritaraiH sArdhamahobhiH sadvigarhitaiH || 46 ||

hk transliteration by Sanscript