Hadith: Allah said: All the deeds of the son of Adam are for himself except for fasting, as it is, indeed, for Me, and I give reward for it

عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«قَالَ اللَّهُ: كُلُّ عَمَلِ ابْنِ آدَمَ لَهُ، إِلَّا الصِّيَامَ، فَإِنَّهُ لِي وَأَنَا أَجْزِي بِهِ، وَالصِّيَامُ جُنَّةٌ، وَإِذَا كَانَ يَوْمُ صَوْمِ أَحَدِكُمْ فَلاَ يَرْفُثْ وَلاَ يَصْخَبْ، فَإِنْ سَابَّهُ أَحَدٌ أَوْ قَاتَلَهُ، فَلْيَقُلْ إِنِّي امْرُؤٌ صَائِمٌ، وَالَّذِي نَفْسُ مُحَمَّدٍ بِيَدِهِ، لَخُلُوفُ فَمِ الصَّائِمِ أَطْيَبُ عِنْدَ اللَّهِ مِنْ رِيحِ المِسْكِ، لِلصَّائِمِ فَرْحَتَانِ يَفْرَحُهُمَا: إِذَا أَفْطَرَ فَرِحَ، وَإِذَا لَقِيَ رَبَّهُ فَرِحَ بِصَوْمِهِ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 1904]

Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Allah said: All the deeds of the son of Adam are for himself except for fasting, as it is, indeed, for Me, and I give reward for it. Fasting is a shield. When it is the day when any of you is fasting, let him not engage in obscenity, and let him not speak in a loud voice. If anyone insults him or fights with him, let him say: Verily, I am a fasting person. By the One in Whose Hand the soul of Muhammad is, the smell of the mouth of the fasting person is more pleasant in the sight of Allah than the smell of musk. The fasting person will rejoice twice: when he breaks his fast, he rejoices, and when he meets his Lord, he rejoices at his fast."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 1904]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed that Allah Almighty says in the Qudsi Hadīth:
Every righteous deed offered by the son of Adam is multiplied by ten up to seven hundred times, except for fasting, which is for Me, as it does not involve ostentation, and I give reward for it. So, none but Me knows how great its reward is and how many times it is multiplied.
Then he said: (Fasting is a shield), protection, shelter, and a fortified fort against the Fire because it is all about abstaining from desires and committing sins, and the Fire is surrounded with desires.
(When it is the day when any of you is fasting, let him not engage in obscenity) at all through sexual intercourse and foreplay or through any obscene speech.
(And let him not speak in a loud voice) by quarreling and shouting.
(If anyone insults him or fights with him) in Ramadan, let him say: I am a fasting person; perhaps this will deter him. However, if he insists on real fighting, he should expel him with the least violent means first.
Then the Prophet (may Allah's peace and blessings be upon him) swore by the One in Whose Hand that his soul is that the foul breath of the fasting person resulting from fasting is more pleasant than the smell of musk in the sight of Allah on the Day of Judgment and is a means of receiving a greater reward than that of the musk, which is recommended to be used on Fridays and during the Dhikr (remembrance of Allah) sessions.
The fasting person will have two occasions to rejoice: when breaking his fast, he rejoices at breaking his fast that eliminates his hunger and thirst; and he rejoices at completing his fast and accomplishing his act of worship, at the alleviation from his act of worship to his Lord, and at the strength it provides him with for his future fasting.
(And when he meets his Lord, he rejoices at his fast), i.e., at its recompense and reward.