“Fasting is not merely abstaining from food and drink; rather, fasting is abstaining from idle talk and indecent speech. If someone reviles you or behaves ignorantly toward you, then say: ‘Indeed, I am fasting.’”

Narrated by Ibn Khuzaymah (no. 1996), Ibn Ḥibbān (no. 3479), and al-Ḥākim (no. 1570), with the wording being his, and by al-Bayhaqī in al-Sunan al-Kubrā (no. 8385), from the ḥadīth of Abū Hurayrah—may Allah be pleased with him.
Authenticated in Ṣaḥīḥ al-Jāmiʿ (no. 5376) and Ṣaḥīḥ al-Targhīb wa-l-Tarhīb (no. 1082).

Brief Explanation of the Hadith

Fasting was legislated to realize piety, to refine souls, and to restrain the limbs from sins—not merely to abstain from food and drink. Rather, among its greatest objectives is safeguarding the limbs, foremost among them the tongue. This is indicated by his statement—may Allah’s peace and blessings be upon him— “Fasting is not merely abstaining from food and drink,” meaning that the complete fasting which brings about great reward is not merely refraining from things that break the fast; rather, “fasting is abstaining from idle talk and indecent speech.”
Idle talk (laghw) is every false statement in which there is no good, such as insult, quarrelling, or disputation without right. As for indecent speech (rafath), it is obscene language and explicit mention of that whose mention is considered offensive and which arouses desire.
This is not to be understood as permitting idle talk or indecent speech outside of fasting, for both are prohibited in all circumstances; rather, the prohibition is more emphatic during fasting because they undermine its purpose and its fruit.
The Prophet—may Allah’s peace and blessings be upon him—then directed to a great etiquette when one is subjected to harm, which is among the requisites of restraining the tongue and through which the effect of fasting upon conduct and character becomes manifest. He said: “If someone reviles you or behaves ignorantly toward you.” Here, ignorance does not mean the opposite of knowledge; rather, it refers to foolishness and poor conduct. It has also been said that it refers to improper speech and actions, such as transgression, harm, or obscenity. “Then say: ‘Indeed, I am fasting,’” whether in one’s heart, on one’s tongue, or both. This serves as a reminder to oneself of the sanctity of the act of worship, a means of restraining the soul from responding in kind, and a clarification of the reason for refraining from reply—so that the aggressor may desist from his harm.
And in the ḥadīth there is an indication of the excellence of Prophetic guidance in refining conduct, an emphasis on the great status of this act of worship, and the obligation of safeguarding its reward. This applies to both obligatory and voluntary fasting.