Defining Fasting:

Linguistically

‘Ṣawm’ (pl. ṣiyām) means abstinence (الإمْسَاكُ).

Legally

إمْسَاكٌ مَخْصُوصٌ في زَمَنٍ مَخْصُوصٍ مِنْ الفَجَرِ الصَادِقِ إلى غُرُوْبِ الشَّمْسِ بِنِيَةٍ مَخْصُوصَةٍ

Withholding from something specified, during a specified time which is from the true fajr until sunset, with a specific intention.

The Merit of Ramadan and Fasting:

Ramadan is the month of the Qur’an, as it was revealed in this month.

Ramadan is the (month) in which was sent down the Qur’an, as a guide to mankind, also clear (signs) for guidance and judgement (between right and wrong)…

(Sūrat al-Baqarah, 2:185)

In it the gates of paradise are opened, the gates of Hell closed, and devils restrained:

حَدِيثُ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ : أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ ( إِذَا جَاءَ رَمَضَانُ فُتِّحَتْ أَبْوَابُ الْجَنَّةِ وَغُلِّقَتْ أَبْوَابُ النَّارِ وَصُفِّدَتِ الشَّيَاطِينُ )  متفق عليه

Abū Hurayrah  reported: Allah’s Messenger (s) said: When there comes the month of Ramadan, the gates of mercy are opened, and the gates of Hell are locked and the devils are chained (al-Bukhārī & Muslim).

Fasting is a shield or protection from the fire and from committing sins

Fasting is an act of worship that is highly rewarded by Allah, this is confirmed in many narrations.

حديث أبِي هُرَيْرَةَ رضي الله عنه, قَالَ: قَالَ رَسُولُ اللهِ صلى الله عليه وسلم:  قَالَ اللهُ: كُلّ عَمَلِ ابْنِ آدَمَ لَهُ إِلاّ الصّيَامَ, فَإِنّهُ لِي وَأَنَا أَجْزِي بِهِ, وَالصّيَامُ جُنّةٌ, وَإِذَا كَانَ يَوْمُ صَوْمِ أَحَدِكُمْ فَلاَ يَرْفُثْ وَلاَ يَصْخَبْ, فَإِنْ سَابّهُ أَحَدٌ أَوْ قَاتَلَهُ فَلْيَقُلْ إِنّي امْرُؤٌ صَائمٌ, وَالّذِي نَفْسُ مُحَمّدٍ بِيَدِهِ لَخُلُوفُ فَمِ الصّائمِ أَطْيَبُ عِنْدَ اللهِ مِنْ رِيحِ الْمِسْكِ لِلصّائمِ فَرْحَتَانِ يَفْرَحُهُمَا: إِذَا أَفْطَرَ فَرِحَ, وَإِذَا لَقِيَ رَبّهُ فَرِحَ بِصَوْمِهِ  أخرجه البخارى

Narrated Abū Hurayrah that Allah’s Messenger (s) said, ‘Allah said, “All the deeds of Ādam’s sons (people) are for them, except fasting which is for Me, and I will give the reward for it. Fasting is a shield or protection from the Fire and from committing sins. If one of you is fasting, he should avoid sexual relations with his wife and quarrelling, and if somebody should fight or quarrel with him, he should say, ‘I am fasting.’ By Him in whose Hands my soul is! The smell coming out from the mouth of a fasting person is better to Allah than the smell of musk. There are two pleasures for the fasting person, one at the time of breaking his fast, and the other at the time when he will meet his Lord; then he will be pleased because of his fasting.’ (al-Bukhārī)

A gate in Paradise called al-Rayyān, for those who observe fasts

حديث سَهْلٍ رضي الله عنه, عَنِ النّبِيّ صلى الله عليه وسلم, قَالَ:  إِنّ فِي الْجَنّةِ بَابًا يُقَالُ لَهُ: الرّيّانُ, يدْخلُ مِنْهُ الصّائمُونَ يَوْمَ الْقِيَامَةِ, لاَ يَدْخُلُ مِنْهُ أَحَدٌ غَيْرُهُمْ, يُقَالُ: أَيْنَ الصّائمُونَ, فَيَقُومُونَ, لاَ يَدْخُلُ مِنْهُ أَحَدٌ غَيْرُهُمْ, فَإِذَا دَخَلُوا أُغْلِقَ فَلَمْ يَدْخُلْ مِنْهُ أَحَدٌ  أخرجه البخارى

Narrated Sahl, the Prophet (s) said, ‘There is a gate in Paradise called al-Rayyān, and those who observe fasts will enter through it on the Day of Resurrection, none except them will enter through it. It will be said, “Where are those who used to observe fasts?” They will get up, none except them will enter through it. After their entry the gate will be closed and nobody else will enter through it.’ (al-Bukhārī)

Fasting Ramadan is an expiation for previous sins

It was narrated from Abū Salamah that Abū Hurayrah said: ‘The Messenger of Allah (s) said: “Whoever fasts Ramadan out of faith and seeking reward, his previous sins will be forgiven”’ (al-Bukhārī).