Musannaf Ibn Abi Shayba 1:1792

Abu Bakr narrated to us, he said: Khalid ibn Makhlad narrated to us, he said: Ibrahim ibn Isma'il ibn Abi Habibah narrated to us, he said: Dawud ibn al-Husayn informed me, from al-Qasim ibn Muhammad, from 'Aishah, she 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."