Hadith 4:299
Bishr ibn al-Mufaddal narrated to us, from Ali ibn Zayd, who said: Sa'id ibn al-Musayyib was asked about the tooth stick (siwak) for the fasting person. He said: There is no harm in it.
حدثنا بشر بن المفضل عن علي بن زيد قال سئل سعيد بن المسيب عن السواك للصائم فقال لا بأس به