Abdur Rahman bin Ghanm (may Allah have mercy on him) said: I asked Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him): Do you use the tooth-stick (miswak) while fasting? He said: Yes. I said: At what time of the day do you use it? He said: At whatever time of the day you wish, in the morning if you wish, or in the evening if you wish. I said: People dislike it in the evening. He asked: Why? I said: They say that the Messenger of Allah (peace and blessings be upon him) said: "The smell of the fasting person's mouth is more pleasing to Allah than the fragrance of musk." So he said: Subhan Allah! When the Messenger of Allah (peace and blessings be upon him) ordered them to use the tooth-stick, he (peace and blessings be upon him) knew that the fasting person's mouth would inevitably have khuluf (a change of odour) - even if he used the tooth-stick - and he (peace and blessings be upon him) was not one to order them to make their mouths foul deliberately. There is no good in that; rather, there is harm in it, except for one who is afflicted with a trial he cannot avoid. I said: And is the dust in the path of Allah likewise? Is only the one rewarded for it who is compelled to it and finds no way to escape it? He said: "Yes, and whoever deliberately throws himself into hardship has no reward in it."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17269