1) What is the best voluntary fast for a Muslimah? I heard of the fast of Dawud Alayhi-salam and that it is the best of fasts.
2) Also, is it haram to voluntarily fast while menstruating?
الجواب حامدا ومصليا ومسلما
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Besides the Fard (obligatory) fasting of Ramadan, there are various Nafl (optional) fasts that Nabi (ﷺ) kept throughout the year. We can also endeavour to perform these fasts sincerely in order to gain maximum reward.
The following fasts are classified as Mustahab (meritorious):
1. Every Monday and Thursday. 
Virtue: Nabi (ﷺ) chose to fast these days because the deeds of the Ummah are presented to Allah Ta’ala on these 2 days. 
2. The 13th, 14th and 15th of every month which are known as Ayyamul beedh. 
Virtue: One who observes these fasts every month will be rewarded as if he fasted his entire lifetime. 
Note: The 13th of the month of Dhul-Hijjah is excluded. 
3. Any three days of each month. 
Virtue: The reward of this fast is same as the previous. In fact this one could be incorporated in the Ayyaamul beedh, as is common.
4. The 9th of Dhul-Hijjah which is the day of Arafah. However, the Haji who is in Arafah is excluded, because he may be weakened through the fast. If this is not the case, then the Haji may also keep this fast. Some ‘Ulama have classified this as a Sunnah fast just like the 10th of Muharram. 
Virtue: By fasting on this day, two years minor sins will be forgiven. 
5. Any six days of Shawwal besides the first of Shawwal; Eid day. 
For more information, refer to the following Fatwa: A Brief Guide to the Six Fasts of Shawwāl
Virtue: The one who observes these fasts after observing the fast of Ramadan, will receive the reward of fasting for the entire year. 
8. As much as possible in the first half of the month of Sha’ban with no specification, on condition that one is confident that this will not weaken him for the fast of Ramadan. 
10. As much as possible, in the month of Muharram.
Virtue: The best fast after Ramadan is the fast of Muharram. 
These are most of the fasts that Rasulullah (sallallahu ‘alayhi wasallam), either observed himself or encouraged others to observe. Since fasting is an act of worship in itself, the Fuqaha have stated that it is virtuous for one to fast as much as possible (if he has the strength to do so).
Most Beloved Fast in the sight of Allah
Rasulullah (ﷺ) is reported to have said that the most beloved fasts to Allah is the fasting arrangement of Dawud (‘alayhis salam); he would fast every alternate day. 
Nonetheless, the key aspect to consider is that one must not subject themselves to undue difficulty beyond their strength and ability. Thus, we advise that one performs the recommended fasts above and then gradually builds up. Thereafter, if they are able to act on the fast of Dawud (‘alayhis salam), they should do so. In doing so, the Muslimah must consult her husband and take his permission.
Note: It is forbidden to fast on the days of both Eids as well as the 11th, 12th, and 13th of Dhul-Hijjah. 
2) It is indeed Harām (impermissible) to fast during menses. If one fasts, the fast will not be valid, regardless of whether it is a Nafl or obligatory fast. If it is an obligatory fast, a Qadhā (replacement) may be necessary in some cases. 
And Allah Ta’āla Knows Best.
[Mufti] Mubasshir Talha
19 Dhul Qa’dah 1442 / 29 June 2021
Concurred by: Annas Shah
Approved by: Mufti Tahir Wadee| Mufti Yusuf Ilolvi
[To see their profiles, click here]
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[Marāqi al-Falāh ma’a Hashiyāh Al-Tahtāwi, 639, DKI]
[Sahih Muslim, Hadīth: 2742 – Sahih ibn Khuzaymah, Hadīth: 2116-2120)
[Marāqi al-Falāh ma’a Hashiyāh Al-Tahtāwi, 639, DKI – Raddul Muhtār – 2/375, HM Saeed]
[Sunan al-Tirmidhi, Hadith: 762 – Sahih ibn Khuzaymah, Hadith: 2128 – Musnad Ahmad, Hadith: 7639]
اس ماه میں ایام بیض کے روزے نہیں ہوسکتے۔ بطور بدل کے خواہ ١٦ کو رکھ لے یا بعد میں رکھلے سب برابر ہیں
[Imdād al-Fatāwa, 4/177, Nu’māniyyah Jadid; 2/102 – Qadeem]
[Hashiyāh Al-Tahtāwi ala ‘l Marāqi al-Falāh, 639, DKI – Raddul Muhtār – 2/375, HM Saeed]
[Sahih Bukhari, Hadith: 1981; Sahih Muslim, Hadith: 2736, 2739]
[Raddul Muhtār – 2/375, HM Saeed]
[Sahih Bukhari, Hadith: 1988; Sahih Muslim, Hadith: 2738-2739]
[Hashiyāh Al-Tahtāwi ala ‘l Marāqi al-Falāh, 639, DKI]
[Sahih Muslim, Hadith: 2750; Sahih ibn Hibban, Hadith: 3634; Sahih ibn Khuzaymah, Hadith: 2114]
[Sahih Bukhari, Hadith: 1969; Sahih Muslim, Hadith: 175, 176, 1156]
الصوم في هذه الأيام مندوب إليه، وهو قياس صوم عاشوراء وصوم شعبان
[Al-Mabsūt, 3/92, Dār al-Ma’rifah]
[Fatāwa al-Hindiyyah, 1/201, Dār al-Fikr]
Hafsah (may Allah be pleased with her) stated: “There are four deeds the Prophet ﷺ never abandoned: Fasting ‘Ashurah, the ten days (of Dhū al-Ḥijjah) and three days of each month, and praying two rakʿat before Fajr Salah” [Sunan al-Nasa’i, Hadith: 2416; Sahih ibn Hibban, Hadith: 6422]
[Sunan al-Tirmidhi, Hadith: 758; Sunan Ibn Mājah, Hadith: 1728]
[Sahih Muslim, Hadith: 2748]
[Sahih Bukhari, Hadith: 1131; Sahih Muslim, Hadith: 2731-2732]
[Raddul Muhtār – 2/375, HM Saeed, Sahih Bukhari, Hadith: 1990; Sahih Muslim, Hadith: 2666-2672; Sahih ibn Khuzaymah, Hadith: 2100; ]
قوله: وصوما أي يحرمه ويمنع صحته
[Raddul Muhtār – 1/291, HM Saeed]
[Al-Nahr al-Fa’iq, 1/130, DKI]
[1/204, Dār al-Kitāb al-Islāmi]