Praise be to Allah.
Siwaak is Sunnah, and is an act of worship when done at the time of prayer or when doing wudoo’, because the Messenger (peace and blessings of Allaah be upon him) said: “Siwaak is purifying for the mouth and pleasing to the Lord.” Narrated by al-Nasaa’i with a saheeh isnaad from ‘Aa’ishah (may Allaah be pleased with her). And because he (peace and blessings of Allaah be upon him) said: “Were it not that it would be too difficult for my ummah, I would have commanded them to use the siwaak at every prayer.” Saheeh – agreed upon. According to another version: “Were it not that it would be too difficult for my ummah, I would have commanded them to use the siwaak at at every wudoo’.” Narrated by Imam al-Nasaa’i with a saheeh isnaad.
With regard to the hadeeth “Two rak’ahs with siwaak are better than seventy rak’ahs without siwaak”, it is a da’eef (weak) hadeeth, and is not saheeh (sound). The saheeh ahaadeeth are sufficient, praise be to Allaah. End quote.
Majmoo’ Fataawa Ibn Baaz (26/288).
Al-Bayhaqi said concerning it in al-Sunan al-Kubra (1/38): (It is) da’eef and its isnaad is not strong. It was narrated in both marfoo’ and mursal reports. End quote.
Ibn al-Qayyim said in al-Manaar al-Muneef (p. 17): Its isnaad is not strong. End quote.
It was classed as da’eef by al-Nawawi in al-Khalaasah (1/88) and by al-Albaani in Da’eef al-Jaami’ (3519).
And Allaah knows best.