Dr Zakir: There are many people who think that using Siwaak while you are fasting is discouraged. It is based on the Hadith. The same Hadith that I quoted earlier of
Sahih Bukhari Vol. 3 in the Book of Fasting Hadith no. 1904 and 1894, Prophet Muhammad (Sallallahu alaihe wa sallam) said that
"By Allah in whose hand is my soul, the breath of a person who fasts is sweeter to Allah (Subhanahu wa Ta'ala) than the sent of musk"
Now based on this people think that if you use Siwaak, the breath, the bad breath that normally comes from the person who fasts will not be there so Allah would not enjoy the breath and based on this, they think it is discouraged.
In fact we should realize that when a person is using the Siwaak, tooth stick, it does not stop the bad breath which normally comes when a person fasts because when you use the tooth stick, the Siwaak it normally massages the gums and if there are any food particles between the teeth like you have tooth brush, it is somewhat similar to that.
The bad breath of fasting comes from the stomach because no food enters the stomach and that's how it comes so no way does it contradict that.
And furthermore our beloved Prophet Muhammad (Sallallahu alaihe wa sallam) said, its mentioned in
Sahih Bukhari Vol. no. 2 (Book of Jummah) Hadith no. 887, the beloved Prophet said that if it wasn't that
"It wasn't too difficult for my Ummah I would have made it compulsory to use the Siwaak before every prayer."
How Wudu is compulsory so the Prophet said if it wouldn't have been difficult of the Ummah he would have made it compulsory to user the Siwaak every time before prayer. And that means it is a recommended act and if it wasn't good for fasting, he would have mentioned it, like how he mentioned fore excessive sniffing of water.
Hadith of Abu Dawud Vol. no. 2 (Book of Fasting) Hadith no. 2360 where our beloved Prophet said that
"Sniff water excessively through your nose while doing ablution except while fasting"
That means sniffing waster excessively is good but don't do it while fasting because there are dangers it will go in to the throat and enter the stomach.
So here too if it was a disadvantage the Prophet would have said that I would have told my Ummah to use the Siwaak except while fasting. So based on this we realize using Siwaak is a Sunnah, it is a recommended act, it is Mustahab and you should do it. And InshaAllah you will get your rewards.