Musnad Abu Ya'la al-Mawsili 238:1

Muhammad ibn Abd al-Rahman ibn Sahm al-Antaki narrated to us, Abdullah ibn al-Mubarak informed us, Sufyan al-Thawri informed us, from Asim ibn Ubayd Allah ibn Asim ibn Umar ibn al-Khattab, from Abdullah ibn Amir ibn Rabiah, from his father, who said: I saw the Messenger of Allah, may Allah bless him and grant him peace, using the siwak while he was fasting, more times than I could count, or he said: more times than I could enumerate.