Hadith: Glory be to Allah, this comes from the devil. Let her sit in a washtub, and if she sees yellowness on top of the water, she has to take a bath once for the Zhuhr and ‘Asr prayers and another bath for the Maghrib and ‘Ishā' prayers and a third bath for the Fajr prayer, and she may perform ablution in between that (if she needs to)

عن أسماء بنت عُمَيْس رضي الله عنها قالت: قلت: يا رسول الله، إن فاطمة بنت أبي حُبَيْش اسْتُحِيضَتْ -مُنْذُ كذا وكذا- فلم تُصَل فقال رسول الله صلى الله عليه وسلم : «سُبحان الله، إن هذا من الشَّيطان لِتَجْلِسْ في مِرْكَنٍ، فإذا رأت صُفْرَة فوق الماء فلتَغْتَسِل للظهر والعصر غُسْلاً واحدا، وتغتسل للمغرب والعشاء غسلا واحدا، وتغتسل للفجر غسلا واحدا، وتتوضأ فيما بَيْنَ ذلك».
[صحيح] - [رواه أبو داود]

Asmā' bint ‘Umays (may Allah be pleased with her) reported: I said: "O Messenger of Allah, Fātimah bint Abi Hubaysh has been having Istihādah since such-and-such and has not been praying since then." The Messenger of Allah (may Allah's peace and blessings be upon him) said: 'Glory be to Allah, this comes from the devil. Let her sit in a washtub, and if she sees yellowness on top of the water, she has to take a bath once for the Zhuhr and ‘Asr prayers and another bath for the Maghrib and ‘Ishā' prayers and a third bath for the Fajr prayer, and she may perform ablution in between that (if she needs to).'"
[Authentic hadith] - [Narrated by Abu Daoud]

Explanation

Asmā' bint ‘Umays (may Allah be pleased with her) informed the Prophet (may Allah's peace and blessings be upon him) of what happened to Fātimah bint Abi Hubaysh who was experiencing some abnormal blood flow, and that this bleeding has prevented her from praying for a long time. The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Glory be to Allah" out of astonishment and surprise. He was surprised at her not praying, even though the blood was not from her menses; rather, it was a kick from the devil, as mentioned in another Hadīth. "Let her sit in a washtub, and if she sees yellowness on top of the water": the Prophet (may Allah's peace and blessings be upon him) guided her to differentiate between the menstrual blood and the blood outside the menstrual period by sitting in a washtub, which is a large container for washing clothes, and if she sees a yellowish color on top of the water, then this is proof that she has become pure from her menses, because the menstrual blood is thick and dark colored while anything other than that is Istihādah. "She has to take a bath once for the Zhuhr and ‘Asr prayers and another bath for the Maghrib and ‘Ishā' prayers and a third bath for the Fajr prayer": if she sees yellowness on top of the water, she has to take a shower three times a day; once for the Zhuhr and ‘Asr prayers, a second time for the Maghrib and ‘Ishā' prayers, and a third time for the Fajr. "And she may perform ablution in between that": meaning that if she wants to pray another (voluntary) prayer in between those prayers, then only needs to make ablution. In other words, if she nullifies her purification (by urinating, defecating, passing wind, etc.), then she only needs to make ablution and not bathe, because bathing is only specific to the five prayers. This bathing is recommended and not obligatory as mentioned in other Hadīths.