Musnad Abu Ya'la al-Mawsili 111:90

Musa narrated to us, Abd al-Rahman narrated to us, from Sufyan, from Abu Ishaq, from al-Tamimi, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to use the siwak frequently, until we thought, or feared, that it would be revealed concerning him.