Musannaf Ibn Abi Shayba 4:298

Ibn Abi ‘Uyaynah narrated to us, from Ghunyah, from his father, from al-Hakam, that he used to see no harm in the *siwak* (tooth stick) for the fasting person at the beginning of the day. And he said: "It was only disliked for him at the end of the day, after his mouth has changed [in odor], as it is liked that it returns into his interior."