بِسۡمِ ٱللَّهِ ٱلرَّحۡمَٰنِ ٱلرَّحِيمِ
'Abdur-Rahman bin Abu 'Atiq said:
"My father told me: 'I heard 'Aishah say, (narrating) from the Prophet (ﷺ): "Siwak is a means of purification for the mouth and is pleasing to the Lord." [Sunan an-Nasa'i :Hadith 5]
Narrated Anas:
Allah's Messenger (ﷺ) said, "I have told you repeatedly to use the Siwak. (The Prophet (ﷺ) put emphasis on the use of the Siwak.[Sahih al-Bukhari 888]
Any kind of sticks that are cleansing and not harmful may be used instead of siwaak, if it is not available, to clean the mouth and remove dirt from the teeth. This includes modern toothbrushes which are known to be beneficial in this regard.
Modern methods of cleaning the teeth, like toothbrushes and so on, that remove dirt and make the mouth smell good, are acceptable.
There is nothing wrong with using siwak that is flavoured with mint, lemon and so on, so long as they do not cause any harm. But a person who is fasting should avoid using any flavoured siwaak, and should only use natural siwaak when he is fasting.
When to clean mouth:
1. When getting up from sleep
Narrated Hudhaifa:
Whenever the Prophet (ﷺ) got up for Tahajjud prayer he used to clean his mouth (and teeth) with Siwak.[Sahih al-Bukhari 1136, 245 and Muslim, 1/220]
2. Optionally for every Prayer
Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, "If I had not found it hard for my followers or the people, I would have ordered them to clean their teeth with Siwak for every prayer." [Sahih al-Bukhari 887]
3. When entering House
Miqdam b. Shuraih narrated it from his father who said:
I asked A'isha what Allah's Apostle (ﷺ) did first when he entered his house, and she replied: He used tooth-stick (first of all). [Sahih Muslim 253]
Among the etiquette of using siwaak, they mentioned the following:
- That a person should not use the siwaak in front of others or in public, because this is impolite.
- That the siwaak should be washed after use, to get rid of any dirt that may be on it. Aaishah (may Allaah be pleased with her) said: The Prophet (peace and blessings of Allaah be upon him) used to use siwaak, then he would give it to me to wash it. I would use it first, then wash it and give it back to him. (Reported by Abu Dawood, 1/45).
- The siwaak should be kept in a clean place.
More info : https://islamqa.info/en/answers/2577/what-is-the-sunnah-with-regard-to-siwaak