Hadith 4:287
Al-Fadl ibn Dukayn narrated to us, from Abd al-Jalil, who said: Shahr ibn Hawshab narrated to me, saying: Ibn Abbas was asked about the *siwak* (tooth stick) for the fasting person. He said: "Yes, the purification. Use the *siwak* in every state."
حدثنا الفضل بن دكين عن عبد الجليل قال حدثني شهر بن حوشب قال سئل بن عباس عن السواك للصائم فقال نعم الطهور استك على كل حال