In the name of Allah,
Fasting during Ramadan is one of Islam’s pillars. The Qur’an and Sunnah mandate it. Scholars disagree on whether someone who abandons it out of laziness has committed disbelief. The correct judgment is that he has not disbelieved.
Islamically, the fast of Ramadan is an act of worship to Allah, the Most High, which consists of refraining from eating, drinking, and sexual relations from dawn to sunset. Fasting is worshipping Allah through abstaining. He leaves them to worship Allah, not as a habit. Click to know, What are the things that break fasting.
Secondly,
A person who is fasting can use siwak at any time, whether it is before or after the sun’s zenith. This is the view from a group of Salaf, an opinion given to al-Shafi’i, and a narration from Ahmad that all support this point of view. Ibn Taymiyyah, al-Qayyim, al-Shawkani, Ibn Baz, and Ibn Uthaymin also chose it. (May Allah have mercy on all)
Some scholars believe that using toothbrushes after midday is disliked.
The Shafis say that it is makruh to use the tooth stick (siwak) after Dhuhr and till sunset. According to Imam al-Nawawi, it is acceptable in all prayers as it is only disliked after the Asr prayer. They also say it is makruh to rinse out the mouth after zawal, other than in wudhu, for this is like siwak.[i]
The prophet said: Verily, the smell of the mouth of a fasting person is better to Allah than the smell of musk.”[Bukhari 5927]
They claimed that since the Siwak reduces the odor emanating from the mouth of a fasting person, Allah will not accept the odor that emanates from it. The smell that Allah loves comes from the stomach, not the mouth because there has been a long time since the last meal. Leaving dirt in one’s mouth and teeth is not something that Allah loves.
Ibn al-‘Arabi [Maliki] said: “Our Scholars said: There is no authentic hadith regarding the fasting person using siwak, negating it or affirming it. However, the Prophet (PBUH) urged the people to use it before every wudhu and before every Salah unrestrictedly without differentiating between the one fasting and the one who is not. He also recommended using siwak on Yawm-ul-Jumu’ah – without differentiating between the one fasting and the one who is not. And we already presented the ten benefits in purification, and fasting has more right than it (i.e., purification).’’[ii]
The Hanafi book Al Hidaya said:
There is no harm in the use of wet siwak in the morning and evening by the person fasting, due to the words of the Prophet,’’ “The best trait of the person fasting is the Siwak.’’ and he did not give any details. Al-Shafi (r) said it is disapproved in the evening insofar as it does away with the blessed effects of the smell of the mouth..’’[iii]
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
“As for the miswak, it is permissible and there is no difference of opinion concerning that. But they differed as to whether it is makruh after the sun has passed the meridian, and there are two well-known views, both of which were narrated from Ahmad. But there is no shar’i evidence suggesting this to be makruh which can be regarded as an exception from the general meaning of the texts about the miswak.”[iv]
References:
[i] Fiqh al Ibadat, according to the Shaafi school of thought, Hajjah Durriyah Al-Aytah, page 300
[ii] Ahadith about Fasting, Rulings and manners, Abridged, Shaikh Fawzan, page 59
[iii] Al Hidaya Vol 1, Page 326
[iv] Majmoo Al Fatawa, Ibn Taymiyyah vol 25, page 266