Musnad Ahmad 1153:908

'Affan narrated to us, he said: Yazid bin Zuray' narrated to us, he said: 'Abd al-Rahman bin Abi 'Atiq narrated to us, from his father, that he heard 'Aishah narrating to him from the Prophet, peace and blessings of Allah be upon him, he said: "Indeed, the siwak is a purification for the mouth, and a means of pleasure for the Lord."