It is narrated from Sayyiduna 'Amir that I have seen the Prophet (peace and blessings be upon him) using the miswak while fasting so many times that I cannot count.
Hadith Referenceمسند احمد / مسند المكيين / 15688
Hadith Gradingحکم دارالسلام:حسن لغيره ، وهذا إسناد ضعيف لضعف عاصم بن عبيدالله