bad breath caused by fasting has its own privileges in the eyes of Allah SWT. In the hadith it is stated that the Messenger of Allah said: "For the sake of the Essence who takes care of me, really the bad breath of a person who is fasting is more fragrant in Allah's sight than mussel oil."
Imam Muslim and An-Nasa'I narrated a hadith which means: "The breath of a person who is fasting is more fragrant with Allah on the Day of Resurrection."
The two hadiths both explain the virtue of a person who fasts. The breath of a person who is fasting is more fragrant than mussel oil in the sight of Allah and will bring rewards from Allah in the hereafter.
Quoting the book Selected Hadith Qudsi Collection by Shaykh Fathi Ghanim, Al-Mazari said that the sentences in the hadith are majaz and isti'arah. Because, the smell of fragrance is a desirable trait so that it is considered as something good. In contrast to the odor that is often avoided, including the bad breath of people who are fasting.
In fact, as it is known that Allah is Most Holy from the attributes of His creatures, including smelling. It's just that, it has become a habit for many people to think that it smells better. Therefore, the isti'arah language style was created to describe that fasting was a way to get closer to Allah SWT even though it resulted in bad breath.
On the other hand, Imam Al-Bujairimi in Tuhfatul Habib ala Syarhil Khatib explains more about this. "What is meant in qaul 'more fragrant according to Allah' is more fragrant than the smell of mussel oil which is ordered to wear it on Fridays and two Eid prayers or that means the reward is more than the reward of using musk oil on Friday or two holidays. Indeed, kissing is impossible for Allah SWT so that what is meant by "more fragrant according to Allah" is His praise and pleasure for people who are fasting. "