Ata' narrates from Abu Hurayrah (may Allah be pleased with him) that he said: The miswak is for you until Asr; when you have prayed Asr, put it aside. Indeed, I heard the Messenger of Allah (peace and blessings be upon him) say: "The smell of the fasting person's mouth is more beloved to Allah Almighty than the fragrance of musk."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8338