Siwak or brushing your teeth is a sunnah matter which is recommended to be done at any time, especially in conditions of bad breath or when going to pray, this is as stated in the hadith:
لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِى لأَمَرْتُهُمْ بِالسِّوَاكِ عِنْدَ كُلِّ وُضُوءٍ
"If it had not burdened my people, I would have ordered them to use their face every time they performed ablution." (Narrated by Bukhari)
However, the scholars have different opinions if it is done while fasting, some judge it is permissible, while some others are of the opinion that it is makruh under certain circumstances.[1]
1. That allows
Some scholars from the Hanafiyyah and Hanabilah circles are of the opinion that it is absolutely permissible to use siwak during the day of fasting. The argument for this opinion is the generality of the hadiths which mention the virtues of the bewak, including those already mentioned. Al Imam Muhammad bin Hasan Asy Syaibani Rahimahullah said:
قال ابو حنيفة لا باس بالسواك للصائم في اية ساعة من ساعات النهار في اوله وفي آخره
"And Imam Abu Hanifah said that there is nothing wrong with sitting on his face for someone who is fasting at all times of the day, both at the beginning and end of the day."[2]
Al Imam Ibn Qudamah Rahimahullah said:
قال أحمد: لا بأس بالسواك للصائم
"Imam Ahmad said: There is nothing wrong with siwak for someone who is fasting."[3]
Syaikhul Islam Ibn Taimiyyah Rahimahullah:
God bless you
And there is no proper syar'i argument to change the law of having a siwak to be makruh.[4]
2. Makruh under certain conditions
While some scholars are of the opinion that the law of miswak for people who are fasting is disallowed under certain circumstances. The arguments for this opinion are first, that the siwak has the potential to cancel the fast by ingesting the parts used for the siwak. If the goal is to clean the mouth, then it's enough to do it in the morning, after all, after that no one pollutes the mouth again until they open it. The second argument is that there is a hadith which mentions the bad breath of fasting people, in which Rasulullah ﷺ said:
لخلوف فم الصائم أطيب عند الله من ريح المسك
"Really the smell of fasting people's mouth, is more fragrant in the sight of Allah than the smell of musk." (Narrated by Bukhari and Muslim)
The following is an explanation from those who hold this opinion.
Malikiyyah Madhhab
This school of thought is of the opinion that it is permissible to clean the mouth with something dry (such as dry miswak) during the day of fasting, while it is permissible to use something wet such as wet miswak or brushing your teeth with paste then it is makruh.[5]
Ibn Abdi Dar Rahimahullah said:
God bless you
"There is nothing wrong with having a miswak for someone who is fasting all day long according to Imam Malik if his miswak is dry, but he takes it if his miswak is wet."[6]
Shafi'iyyah Madhhab
This school of thought allows miswak without disrepute if it is done in the morning until zawal time, as for after zawal, that is when the midday prayer starts until the sun sets, it is makruh.[7]
Al Imam an Nawawi Rahimahullah said:
قال الشافعي والأصحاب يكره للصائم السواك بعد الزوال هذا هو المشهور ولا فرق بين صوم النفل والفرض... والاستياك قبل الزوال بالرطب واليابر جاب
"Imam Syafi'i and his scholars said that it is forbidden for a person who is fasting to wear a posture after the sun has slipped, and this is a well-known opinion, whether fasting is sunnah or obligatory, as for sitting before zawal, whether with a dry or wet miswak, the law is notdisastrous.”[8]
Part of the Hanbali Madhhab
There are some Hanbali scholars who are of the same opinion as the Shafi'iyyah, that is, it is permissible before zawal and it is makruh after it. Abu Musa al Hasyimi al Hanbali said:
God bless you
“There is nothing wrong with having a siwak for someone who fasts before the zawal time. And he should refrain from performing his limbs after the zawal time."
Conclusion
According to the majority of scholars, it is permissible to use siwak while fasting, while some judge it is makruh if it is time for zawal and use wet ingredients such as brushing your teeth with paste. Al Imam Syaukani said as quoted by Shaykh Wahbah Uhaili Rahimahumallah:
الحق أنه يستحب السواك للصائم أول النهار وآخره، وهو مذهب جمهور الأئمة
“The truth is that it is still sunnah to wear a siwak for people who are fasting, both at the beginning of the day and at the end of it. And this is the school of thought of the majority of the priests.”[9]
But what needs to be remembered, for those who brush their teeth while fasting, should do so carefully, because if you swallow any parts such as toothpaste and water when rinsing your mouth, it can break your fast. Al Imam Nawawi Rahimahullah said:
لو استاك بسواك رطب فانفصل من رطوبته أو خشبه المتشعب شئ وابتلعه أفطر بلا
“If there are people who use miswak with wet miswak. Then there is a part that is separated from the miswak that he uses, or the part (hairs) from the stem falls off and then is swallowed, then his fast is invalidated without any difference of opinion among the scholars."[10]
That is, if you swallow the water and toothpaste, what if you swallow the toothbrush? Yes, if that's the case, it's not just the fast that's canceled, your life will also end. Automatic iftar in Barzakh realm.
Wallahu a'lam.
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[1] Dar al Mukhtar (2/114), al Hidayah (2/270), Al Majmu' Syarah alMuhadzdzab (6/377), al Mughni (3/125), al Qawanin al Fiqhiyyah p. 80.
[2] Hujjah 'ala Expertl Medina (1/411)
[3] Al Mughni (4/359)
[4] Majmu' Fatawa (25/266).
[5] Asy Syarah ash Shaghir (1/292).
[6] Al Kafi fi Fiqhi Ahlil Medina (1/352).
[7] Raudhatut Talibin (2/368).
[8] Al Majmu' Syarah al Muhadzdab (6/377).
[9] Fiqh al Islami wa Adillatuhu (1/457)
[10] Majmu' Syarh al Muhadzdzab (6/318)