It is an act of Sunnah (whatever is reported from the Prophet)
to use Siwak when intending to offer Salah prior to commencing it. This is substantiated by what the Six Hadith Compilers (Al-Bukhari, Muslim, Abu Dawud, Al-Tirmidhy, Al-Nasa’y, and Ibn Majah) narrated on the authority of
Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace be upon him) said:
“If it were not for the fear that I might make it hard for my Ummah (nation based on one creed), I would have commanded them to use the Siwak at the time of every Salah.”
using Siwak after commencing Salah it is impermissible.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.