Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) said: "Were it not that I feared you would be unable to do it, I would have ordered you to use the tooth-stick (siwak) with every prayer."
Al-Bazzar recorded it, and al-Tabarani recorded it in al-Mu'jam al-Kabir through Muslim bin Kaysan al-Mula'i, who is da'if (weak); al-Bazzar said: There is no harm in him.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2549