Hadith: Why does the menstruating woman make up for her missed fasts but does not make up for the missed prayers? She said: Are you a Harūriyyah? I said: I am not a Harūriyyah, but I am just asking. She said: That would happen to us, so we were ordered to make up for the fasts but were not ordered to make up for the prayers.

عن مُعاذة قالتْ: سألتُ عائشةَ رضي الله عنها فقلتُ: «مَا بَال الحَائِضِ تَقضِي الصَّوم، ولا تَقضِي الصَّلاة؟ فقالت: أَحَرُورِيةٌ أنت؟، فقلت: لَستُ بِحَرُورِيَّةٍ، ولَكنِّي أسأل، فقالت: كان يُصِيبُنَا ذلك، فَنُؤمَر بِقَضَاء الصَّوم، ولا نُؤْمَر بِقَضَاء الصَّلاَة».
[صحيح] - [متفق عليه]

Mu‘ādhah reported: 'I asked ‘Ā’ishah (may Allah be pleased with her): "Why does the menstruating woman make up for her missed fasts but does not make up for the missed prayers?" She said: "Are you a Harūriyyah?" I said: "I am not a Harūriyyah, but I am just asking." She said: "That would happen to us, so we were ordered to make up for the fasts but were not ordered to make up for the prayers."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Mu‘ādhah asked ‘Ā’ishah (may Allah be pleased with her) why women make up for the fasts which they miss during their menstruation but do not make up for the prayers, although both fasting and praying are mandatory, and prayer is even greater than fasting. The deviant sect of Kharijites (also called Harūriyyah) were known for their strict approach to this question, and thus did not discriminate between the two acts of worship (i.e. they held that both prayer and fasting must be made up). Therefore, ‘Ā’ishah asked Mu‘ādhah if she were such a hardliner. She replied that she was not, but only asked to learn and seek guidance. So ‘Ā’ishah said that when women had their menstrual cycle during the life of the Prophet (may Allah's peace and blessings be upon him) they would stop praying and fasting during menstruation, and he would command them to make up for the fasts they missed but did not command them to make up for the missed prayers. Had making up for prayers been obligatory, he would have commanded it and not remain silent about it. It is as if ‘Ā’ishah is telling her: It is sufficient as wisdom and guidance to comply with the commands of Allah and His Messenger and abide by the limits they set.