All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The wisdom behind the ordainment of fasting is to attain piety. Allaah says (what means): {O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous.} [Quran 2: 183] The Prophet, sallallaahu ‘alayhi wa sallam, described the situation of some fasting people in the following way, “It may be that a fasting person attains nothing but hunger and thirst from his fast.” [Ahmad and Al-Haakim]
The Prophet, sallallaahu ‘alayhi wa sallam, has strictly forbidden insulting others and all kinds of ill conduct. The Prophet, sallallaahu ‘alayhi wa sallam, said, “If a person is fasting, then he should avoid foul language and ill conduct. If a fasting person is cursed or fought, then he should answer, ‘I am fasting. I am fasting.’” [Al-Bukhari and Muslim] The Prophet, sallallaahu ‘alayhi wa sallam, also says, “If a fasting person does not give up false speech and action, then Allah does not need him to give up his food and drink.” [Al-Bukhari and Muslim]
Therefore, one who is fasting is obligated to abstain from such forbidden acts. If he commits something thereof, then he must turn to Allaah in repentance. Nevertheless, none of these forbidden acts invalidates one's fast, neither in Ramadan nor at any other time, because fasting means to abstain from food, drink, sexual intercourse, and the like from dawn to sunset. As long as the fasting person avoids the actions that invalidate the fast, his fast is valid.
Allaah Knows best.