Musnad Abu Ya'la al-Mawsili 150:94

Muhammad ibn al-Sayyah narrated to us, Ismail narrated to us, from Muhammad ibn Ishaq, from Abdullah ibn Muhammad ibn Abi Atiq, who said: I heard Aisha saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "The siwak is a purification for the mouth and a means of pleasure for the Lord."