أَقَلُّ الْحَيْضِ ثَلَاثَةُ أَيَّامٍ وَلَيَالِيهَا، وَمَا نَقَصَ مِنْ ذَلِكَ فَلَيْسَ بِحَيْضٍ، وَهُوَ اسْتِحَاضَةٌ.
Meaning: The minimum menstrual period is three days with their nights; bleeding shorter than that is not hayd but istihadah.
The three days include their nights, rather than merely any three calendar dates. Bleeding shorter than the Hanafi minimum is classified as istihadah, not menstruation.
This is the Hanafi threshold; other schools use different criteria for a minimum period.
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Menstruation: Duration, Worship, Cessation and Intervening Purity
أَقَلُّ الْحَيْضِ ثَلَاثَةُ أَيَّامٍ وَلَيَالِيهَا، وَمَا نَقَصَ مِنْ ذَلِكَ فَلَيْسَ بِحَيْضٍ، وَهُوَ اسْتِحَاضَةٌ.
Meaning: The minimum menstrual period is three days with their nights; bleeding shorter than that is not hayd but istihadah.
The three days include their nights, rather than merely any three calendar dates. Bleeding shorter than the Hanafi minimum is classified as istihadah, not menstruation.
This is the Hanafi threshold; other schools use different criteria for a minimum period.