If one announces Takbirat-ul-Ihram (saying: "Allahu Akbar [Allah is the Greatest]" upon starting Prayer) in a supererogatory prayer and forgets to cleanse their teeth with Siwak (tooth-cleansing stick), are they allowed to use it upon remembering it?
It is permissible to cleanse one's teeth prior to Takbirat-ul-Ihram. This is based on the authentic Hadith in which the Prophet (peace be upon him) is reported to have said,
Were it not for the fear that it might be hard for my Ummah (nation), I would have ordered them to use Siwak (tooth-cleansing stick) at every Salah (prayer).
If one forgetfully starts Salah without first cleansing their teeth, they do not have to do it as they will be doing an act not sanctioned by Sunnah.
Q 7: If the Imam announces Takbirat-ul-Ihram (saying: "Allahu Akbar [Allah is the Greatest]" upon starting Prayer) while there is a person reciting the Glorious Qur'an in the Masjid (mosque), does this person have to stop reciting and join the prayer? Can this person use Siwak (tooth-cleansing stick) during the time when the Imam recites Du`a'-ul-Istiftah (opening supplication when starting the Prayer)? (Part No. 6; Page No. 333)
It is an act of Sunnah that this person stops recitation upon hearing the Iqamah (call to start the Prayer) and joins the prayer with others. It is also an act of Sunnah that he cleanses his teeth with Siwak before starting the prayer. It is not permissible for him to use Siwak after the Imam has announced Takbirat-ul-Ihram. He should immediately follow the Imam in prayer. This is based on the generality of the Hadith on the necessity of following the Imam immediately when he starts prayer. One Hadith narrated in this regard states,
The Imam is appointed so that he should be followed, so do not be at variance with him. Recite takbir when he recites it...
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.