Mu'adha said: I asked 'A'isha: What is the reason that a menstruating woman completes the fasts (that she abandons during her monthly course). but she does not complete the prayers? She (Hadrat 'A'isha) said: Are you a Haruriya? I said: I am not a Haruriya, but I simply want to inquire. She said: We passed through this (period of menstruation), and we were ordered to complete the fasts, but were not ordered to complete the prayers.
Source: sunnah.com
Other translations
Arabic
وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنْ عَاصِمٍ، عَنْ مُعَاذَةَ، قَالَتْ سَأَلْتُ عَائِشَةَ فَقُلْتُ مَا بَالُ الْحَائِضِ تَقْضِي الصَّوْمَ وَلاَ تَقْضِي الصَّلاَةَ فَقَالَتْ أَحَرُورِيَّةٌ أَنْتِ قُلْتُ لَسْتُ بِحَرُورِيَّةٍ وَلَكِنِّي أَسْأَلُ . قَالَتْ كَانَ يُصِيبُنَا ذَلِكَ فَنُؤْمَرُ بِقَضَاءِ الصَّوْمِ وَلاَ نُؤْمَرُ بِقَضَاءِ الصَّلاَةِ .
sunnah.com