Musnad Abu Ya'la al-Mawsili 110:3

Bishr ibn al-Walid narrated to us, Sharik narrated to us, from Abu Ishaq, from al-Tamimi, from Ibn Abbas, from the Prophet (peace be upon him), who said: "I have been commanded to use the siwak (tooth stick) until I thought that a Quran or revelation would be sent down to me regarding it."