It was narrated on the authority of Abu Hurayrah, may Allah be pleased with him, that the Prophet said: "Fasting is a shield (from Hell)." [Al-Bukhari and Muslim]
Another narration by Ahmad reads: “Fasting is a shield and a strong fort (to safeguard one) from Hell.”
It was narrated on the authority of Jaabir, may Allah be pleased with him, that the Messenger of Allah said: “Fasting is a shield wherewith the slave (fasting person) shields himself from Hell.” [Ahmad]
It narrated on the authority of ‘Uthmaan ibn Abu Al-‘Aas Ath-Thaqafi, may Allah be pleased with him, that he said: “I heard the Messenger of Allah say: ‘Fasting is a shield from Hell, like the shield of anyone of you from (the dangers of) fighting.’" [Ahmad, An-Nasaa’i and Ibn Maajah]
It was narrated on the authority of Abu ‘Ubaydah ibn Al-Jarraah, may Allah be pleased with him, that he said, “I heard the Messenger of Allah say: ‘Fasting is a shield as long as one (the fasting person) does not make a hole in it (i.e. violate it).’" [Ahmad, An-Nasaa’i, At-Tayaalisi and Abu Ya‘la]
Benefits and rulings
First: Fasting is a means to curb desires that would lead their perpetrators to Hell. This is why it acts as a screen from the Fire. Al-‘Iraaqi said, “Fasting is a protective shield because it means abstaining from desires, and the Fire is surrounded by desires.”
Second: The virtue of fasting. Thus, the Muslim should observe much voluntary fasting if he has power to endure it, provided that it would not divert him from more preferable deeds such as the Jihaad and the like.
Third: The fasting that protects and screens from the Fire is that which is unstained with what would remove or reduce its reward, like backbiting, talebearing, telling lies and using foul words, as confirmed in the abovementioned narration of Abu ‘Ubaydah, may Allah be pleased with him: "Fasting is a shield as long as one (the fasting person) does not make a hole in it (i.e. violate it)." Making holes in it would be by doing unlawful things. Hence, the fasting person is required to guard his fasting from all unlawful deeds that would remove and reduce its reward, so that it may indeed be a screen for him from the Fire (of Hell).
Fourth: The purpose of fasting is to purify souls and mend hearts, and not just to abstain from food and drink.