Ghusl for Menstruation and Postpartum
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 21
وَ قَدْ رُوِيَ: «أَنَّهُ صَارَ حَدُّ قُعُودِ اَلنُّفَسَاءِ عَنِ اَلصَّلاَةِ ثَمَانِيَةَ عَشَرَ يَوْماً لِأَنَّ أَقَلَّ اَلْحَيْضِ ثَلاَثَةُ أَيَّامٍ وَ أَكْثَرَهُ عَشَرَةُ أَيَّامٍ فَأَوْسَطُهُ خَمْسَةُ أَيَّامٍ فَجَعَلَ اَللَّهُ عَزَّ وَ جَلَّ لِلنُّفَسَاءِ أَيَّامَ أَقَلِّ اَلْحَيْضِ وَ أَوْسَطِهِ وَ أَكْثَرِهِ».
It has been narrated: 'The limit for the postnatal period of refraining from prayer has been set at eighteen days because the minimum duration of menstruation is three days, the maximum is ten days, and the average is five days. Thus, Allah سبحانه وتعالى, the Exalted, has designated the days of postnatal bleeding (nifas) to correspond with the minimum, average, and maximum days of menstruation.'
- Ṣaḥīḥ al-Faqīh