Musannaf Ibn Abi Shayba 1:1809

Waki’ narrated to us, he said: Isra’il narrated to us, from Abu Ishaq, from al-Tamimi, who said: I asked Ibn Abbas about the *siwak* (tooth stick). He said: The Messenger of Allah, may Allah bless him and grant him peace, did not cease commanding it, until we thought that something would be revealed to him concerning it.