Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) reports this saying of the Prophet (peace and blessings be upon him): "The siwak is a cause of purifying the mouth, of the Lord's pleasure, and of sharpening the eyesight."
Imam al-Tabarani recorded this report in al-Mu'jam al-Awsat, and something similar in al-Mu'jam al-Kabir; its chain contains a narrator, Bahr bin Kunayz al-Saqqa', on whose weakness the hadith scholars are agreed.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1115