Sayyiduna Amir bin Rabi'ah al-Adawi (may Allah be pleased with him) narrates: I cannot count how often I saw the Prophet (peace and blessings be upon him) using the miswak (tooth-stick), and he (peace and blessings be upon him) would use the miswak while fasting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8325
Sayyidah Aishah (may Allah be pleased with her) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Among the best qualities of the fasting person is using the miswak."
Others are more reliable than Mujalid, and Asim bin Ubaydullah is not strong; and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8326
Abu Ishaq al-Khwarizmi, the judge of Khwarizm, narrates: I asked Asim al-Ahwal: Should a fasting person use the miswak? He said: Yes. I said: With both a fresh and a dry miswak? He said: Yes. I said: At the beginning of the day and at its end too? He said: Yes. I said: From whom is this narrated? He said: From Anas bin Malik (may Allah be pleased with him), from the Prophet (peace and blessings be upon him).
Only Abu Ishaq Ibrahim bin Baytar narrated this; he is also called Ibrahim bin Abd al-Rahman, the judge of Khwarizm. He narrated munkar (denounced) reports from Asim al-Ahwal in Balkh and is not used as evidence. It has also been narrated from him through another route that does not mention the beginning and end of the day.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8327
Ibrahim bin Abd al-Rahman said: I asked Asim al-Ahwal about the miswak for a fasting person, and he said: There is no harm in it. I said: With both a fresh and a dry one? He said: Do you think it is moister than water? I said: From whom did you hear it? He said: From Sayyiduna Anas bin Malik (may Allah be pleased with him), from the Prophet (peace and blessings be upon him).
Abu Ahmad (Ibn Adi) said: Most of the hadiths of this Ibrahim are not preserved (ghayr mahfuzah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8328
Ziyad bin Hudayr narrates: I have not seen anyone more persistent in using the miswak while fasting than Sayyiduna Umar (may Allah be pleased with him). I think he said: with a stick. Abu Ubayd said: meaning a dry stick.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8329
Abdullah bin Nafi', the freed slave of Sayyiduna Ibn Umar (may Allah be pleased with them both), narrates from his father, who narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both), that he used to use the miswak while fasting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8330