Musnad Ahmad 1153:1996

Yazid narrated to us, he said: Muhammad ibn Ishaq informed us, from Abdullah ibn Muhammad ibn Abd al-Rahman ibn Abi Bakr, from Aisha, she said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Indeed, the siwak is a purification for the mouth, and a means of pleasure for the Lord."