Majma al-Zawaid — Hadith 4955

Abd al-Rahman bin Ghanm narrates: I asked Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him): May I use the siwak while I am fasting? He said: Yes. I asked: At what part of the day should I use it? He said: At any part of the day you wish; if you wish, in the morning, and if you wish, in the evening. I said: People consider it disliked in the evening. He asked: Why is that? I replied: People say that the Prophet (peace and blessings be upon him) said: "The smell of the fasting person's mouth is purer in the sight of Allah Almighty than the fragrance of musk." Sayyiduna Mu'adh (may Allah be pleased with him) said: Subhan Allah! Whenever the Prophet (peace and blessings be upon him) commanded the people to use the siwak, he commanded them knowing that the mouth of a fasting person will inevitably have an odour even if he has used the siwak. Nor did the Prophet (peace and blessings be upon him) command people to make their mouths smell deliberately; there would be no good in that, rather it would be an evil, except for the one who is afflicted with a trial and has no way to escape it. I said: Is it the same with dust in the path of Allah, that the one who is compelled to it and finds no escape from it will be rewarded for it? He said: Yes. As for the one who deliberately throws himself into trial, he will receive no reward.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains Bakr bin Khunays, and he is da'if (weak); according to one report, Yahya bin Ma'in declared him thiqah (trustworthy).