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, Muslim)
Some people think fast entails merely abstention from food, drink, and sex. This is simply the physical form of fast. But this is not all Allah expects from believers. The real spirit of fast 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 fast, 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.
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.’ Here also the same idea of attaining taqwa as the real intent of fasting finds expression in a subtle manner. This hadīth warns them against the result of observing what we may call ‘physical fast’. What is expected is a march from ‘physical fast’ to ‘spiritual fast’.