Musnad Ahmad 25:939

Aswad ibn Amir narrated to us, Sharik narrated to us, from Abu Ishaq, from al-Tamimi, from Ibn Abbas, who said: "I have been commanded to use the siwak (tooth stick) to the extent that I thought a Quran or revelation would be sent down to me regarding it." The Prophet, may Allah bless him and grant him peace, is the one who said this.