Musnad Ahmad 1153:323

'Abdah ibn Sulayman al-Kilabi narrated to us, Muhammad ibn Ishaq narrated to us, from 'Abdullah ibn Muhammad, who said: I heard 'Aishah say: The Messenger of Allah, may Allah bless him and grant him peace, said: "The siwak is a purification for the mouth and a means of seeking the pleasure of the Lord, the Almighty and Majestic." 'Abdullah ibn Ahmad said: "'Abdullah ibn Muhammad: it is said of him: Abu 'Atiq."