Musnad Ahmad 1153:2321

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: And Muhammad ibn Muslim ibn Shihab al-Zuhri mentioned, from 'Urwah ibn al-Zubayr, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, from the Prophet, may Allah bless him and grant him peace, that he said: "The excellence of prayer with the siwak over prayer without a siwak is seventy times."