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