Musnad Ahmad 1153:194

Isma'il narrated to us, from Muhammad ibn Ishaq, who said: 'Abdullah ibn Muhammad ibn 'Abd al-Rahman ibn Abi Bakr narrated to me, from 'Aishah, who said: 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."