Musnad Abu Ya'la al-Mawsili 1:107

'Abd al-A'la ibn Hammad al-Narsi narrated to us, he said: And I asked him about it, and he said: "This is a mistake." Then he narrated it to me, he said: Hammad ibn Salamah narrated to us, from Ibn Abi 'Atiq, from his father, from Abu Bakr al-Siddiq, he 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 pleasure for the Lord."