It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), he says: Two men came to the Noble Prophet (peace and blessings be upon him), both had a similar need. When one of them spoke, the Prophet (peace and blessings be upon him) noticed an unpleasant odor from his mouth and asked him: "Do you not use the miswak?" He replied: "Yes, I certainly do, but the real matter is that I have not eaten food for three days." Then the Prophet (peace and blessings be upon him) instructed a man, so he gave him a place and fulfilled his need.
Hadith Referenceالفتح الربانی / كتاب الطهارة / 574
Hadith Gradingمحدثین:ضعیف
Hadith Takhrij«اسناده ضعيف، قابوس بن ابي ظبيان ليِّن يكتب حديثه ولا يحتج به۔ أخرجه الطبراني: 12611، والبيھقي: 1/ 39 ، (انظر مسند أحمد ترقيم الرسالة: 2409 ترقیم بيت الأفكار الدولية: 2409»
Brief Explanation
Benefits: … The general evidences regarding the use of the tooth-stick (miswak) also include the fasting person, while there are also specific evidences present. Furthermore, there is no such evidence in which the fasting person is prohibited from using the miswak. In addition, if it is permissible for the fasting person to rinse the mouth during ablution (wudu), in which water is moved around throughout the mouth, then the use of the miswak should also be permissible. It should be kept in mind that the odor from the mouth of the fasting person—which, in the sight of Allah Ta‘ala, is purer than musk—relates to the stomach of the fasting person, not to the mouth; therefore, the use of the miswak does not affect that odor.