As-Siyaam (fasting) is Junnah (protection or shield or a screen or a shelter from the Hell-fire)

706. Narrated Aboo Huraira (radhi Allaahu anhu):Allaah’s Messenger (Sallallaahu Alaihi Wasallam) said,

As-Siyaam (fasting) is Junnah (protection or shield or a screen or a shelter from the Hell-fire). So, the person observing Saum should avoid sexual relation with his wife and should not behave foolishly and impudently, and if somebody fights with him or abuses him, he should say to him twice, ‘I am observing Saum.’ “

The Prophet (Sallallaahu Alaihi Wasallam) added,

“By Him in Whose Hands my soul is, the smell coming from the mouth of a person observing Saum is better with Allaah (Subhanahu wata’alaa) than the smell of musk. (Allaah says about the fasting person), ‘He has left his food, drink and desires for My sake. The Saum is for Me.(*) So I will reward (the fasting person) for it and the reward of good deeds is multiplied ten times.’ “

[Saheeh Al-Bukhaari, Hadeeth No. 118, Vol. 3]

(*) (H.706) Although all practices  of worshipping are for Allaah Alone, here Allaah  (Subhanahu wata’alaa) singles out Saum (fast), because Saum cannot be practised for the sake of showing off as nobody can know whether one is observing Saum or not, except Allaah. Therefore, fasting is a pure performance that cannat be blemished with hypocrisy. (Fath-ul-Baaree, Vol. 5, p. 10).

Source : The Book Of As-Siyaam (the fasting) – Chapter 29 – To protect one’s tongue (from evil talk) while observing Saum (fast). Al-Lulu Wal-Marjan,Compiled by : Fuwad Abdul-Baqi,Translated by :Dr. Muhammad Muhsin Khan