Would quarreling or using obscene language during fasting invalidate one’s fast?

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It does not necessarily invalidate the fast; it nullifies its reward. The Muslim must observe self-control and refrain from the use of obscene language, slander, backbiting, and actions of the like whilst fasting and at all other times. He/she must be more keen than usual to observe these guidelines while fasting in order to earn the complete reward of the day’s fast.