It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that the Messenger of Allah (peace and blessings be upon him) said: "The prayer for which the siwak is used is seventy degrees superior to the prayer for which the siwak is not used." (This hadith is one of those about which it is feared that they are among the tadlis (concealed omissions) of Muhammad bin Ishaq bin Yasar, and that his hearing it from Imam al-Zuhri is not established.)
Mu'awiyah bin Yahya al-Sadafi also narrated it from al-Zuhri, but he is not strong. It has also been narrated through another route from Urwah from Aishah, and through another route from Amrah from Aishah, and both of them are da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 159