Musnad Abu Ya'la al-Mawsili 1:108

Abu Khaythama narrated to us, Yunus ibn Muhammad narrated to us, Hammad ibn Salama narrated to us, from Ibn Abi Atiq, from Abu Bakr al-Siddiq, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The siwak is a purification for the mouth and a means of pleasure for the Lord."