Musnad Abu Ya'la al-Mawsili 150:412

'Abd al-A'la narrated to us, 'Abd al-'Aziz bin Muhammad al-Darawardi also narrated to us, from Ibn Abi 'Atiq, from his father, from 'Aishah, that the Prophet (peace and blessings of Allah be upon him) said: "The siwak is a purification for the mouth and a means of pleasure for the Lord."