Abū Hurairah reported: The Messenger of Allah (peace and blessings of Allah be to him) said, “When any one of you is observing a fast on a day, he should neither indulge in obscene language nor should he raise his voice; and if anyone reviles him or tries to quarrel with him he should say: ‘I am observing fast’.”
(Bukhari and Muslim)
Some people think that fast entails merely abstention from food, drink, and sexual intercourse. This is of course the physical form of fast. But this is not all that Allah expects from fast-observing believers. The real spirit of fast, as the Holy Qur'an stipulates, is attainment of taqwa (piety) – la’allakum tattaqoon (so that you may become God-fearing. So when a believer is in a state of observing fasts for the Pleasure of Allah, he is required to abstain from using obscene or foul language, false conduct, quarrels, disputes, etc. And if someone reviles him or tries to pick a quarrel with him, he should politely declare: ‘I am observing fast (for the Pleasure of Allah)’ and leave that person alone. Thereby he can keep himself onto the path of piety and enjoy the blessings of fasting and closeness of the Lord.
In another narration the Messenger of Allah (peace and blessings of Allah be to him) declared: ‘Some people get nothing (no rewards) for their fasts except hunger and thirst.’ In this hadīth also the same idea of attaining taqwa as the real intent of fasting finds expression in a subtle manner. This hadīth in fact tells the believers and aims at warning them against the result of observing what we may call ‘physical fast’; for the observers of such fasts would not get any reward for their ‘hardship’ – abstaining from hunger and thirst. What is expected is a march from ‘physical fast’ to ‘spiritual fast’.