Praise be to Allah.
Firstly:
Allah has prescribed fasting for great reasons. One of the most important of these reasons and one of the greatest purposes achieved by the fast is so that we may attain piety (taqwa), as Allah says (interpretation of the meaning) :
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious)” [al-Baqarah 2:183]
Taqwa or piety means obeying that which Allah has commanded and avoiding that which He has forbidden.
The person who is fasting is commanded to do acts of worship and obedience, and he is forbidden to do haraam things. Sins are bad enough when committed by anyone, but they are even worse when committed by one who is fasting. Hence the Prophet (peace and blessings of Allah be upon him) said: “Whoever does not give up false speech and acting in accordance with it, and ignorant conduct, Allah has no need of him giving up his food and drink.” Narrated by al-Bukhari, 6057. See also question no. 37989 and 37658.
Ibn Khuzaymah, Ibn Hibaan and al-Haakim narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Fasting does not just mean giving up food and drink, rather fasting also means giving up idle and obscene talk.” Classed as saheeh by al-Albaani in Saheeh al-Targheeb wa’l-Tarheeb, 1082.
‘Umar ibn al-Khattaab and ‘Ali ibn Abi Taalib (may Allah be pleased with them) said: Fasting does not just mean giving up food and drink; it also means giving up lies, falsehood and idle talk.
Jaabir ibn ‘Abd-Allah said: If you fast, then block your ears, eyes and tongue from lies and sin; do not abuse your servants; be tranquil and dignified on the day of your fast; do not let the day when you do not fast and the day when you fast be the same.
It was narrated that Taleeq ibn Qays said: Abu Dharr said: When you fast then be careful as much as you can. When Taleeq was fasting he would enter his house and only come out to pray.
When Abu Hurayrah and his companions (may Allah be pleased with them) fasted, they would stay in the mosque and say, “We are keeping our fast pure.” See al-Muhalla, 4/305
One of the scholars said:
The person who is fasting must fast with his eyes and not look at anything that is not permissible; with his ears and not listen to anything that is not permissible; with his tongue and not speak any obscene words or slander any one or tell lies or backbite.
The believer should make the most of this blessed month in which the devils are chained up, the gates of Paradise are opened and the gates of Hell are closed, and a caller cries out, “O seeker of good, go ahead; O seeker of evil, desist!” The believer should make the most of this month so that he may draw closer to Allah, repent sincerely from all his sins and make a pledge to Allah to adhere to His religion and laws.
Secondly:
Sin – including a woman’s making a wanton display (tabarruj) of her beauty and charms to non-mahram men – detracts from the reward of fasting. The more sins are committed and the more serious they are, the more that detracts from the reward of fasting, and it may cancel out the reward altogether, so that a person may have denied himself food, drink and all the things that break the fast, yet still deprived himself of the reward for that by disobeying Allah. Hence the Prophet (peace and blessings of Allah be upon him) said: “The one who fasts may get nothing from his fast but hunger, and the one who prays qiyaam may get nothing from his qiyaam but a sleepless night.” Narrated by Ibn Maajah, 1690; classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
Al-Subki said in his Fataawa, 1/221-226:
Can the (reward for) fasting be decreased by sins that may occur while one is fasting, or not? The view which we favour is that it can decrease, and I do not think there is any differing opinion on this matter…
Note that the complete reward for fasting is achieved when it is accompanied by acts of worship such as reading Qur’aan, observing i’tikaaf, praying, giving charity etc, and it may be achieved by avoiding haraam things. All of these things make the reward more complete.
Thirdly:
With regard to fasting that is accompanied by sins – including tabarruj on the part of a woman – the fast is not invalidated by that, rather it is valid and the fasting person has fulfilled his obligation and does not have to make up the fast. But the reward for fasting is decreased by that sin and it may be cancelled out altogether as stated above.
Al-Nawawi said in al-Majmoo’ (6/398):
The fasting person should protect his fast from backbiting and slander. What this means is that it is more important for the fasting person to avoid these sins than others, although those who are not fasting should avoid these sins too, as that is enjoined in all situations. If a person backbites while fasting, he has committed a sin but that does not invalidate his fast in our view. This is the view of Maalik, Abu Haneefah, Ahmad and all the scholars except al-Awzaa’i who said that the fast is invalidated by backbiting, and must be made up.
Shaykh Ibn ‘Uthaymeen was asked in Fataawa al-Siyaam (p. 358): If a person utters some haraam words during the day in Ramadaan does that invalidate his fast?
He replied:
If we read the words of Allah (interpretation of the meaning):
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious)” [al-Baqarah 2:183]
we will see that the reason why fasting is enjoined is so that we may attain piety, and piety means avoiding things that are forbidden. In general it means doing that which is commanded and avoiding that which is forbidden. The Prophet (peace and blessings of Allah be upon him) said: “Whoever does not give up false speech and acting in accordance with it, and ignorant conduct, Allah has no need of him giving up his food and drink.”
Based on this, the fasting person has to avoid haraam words and actions, so he should not backbite about people, or tell lies, or spread malicious gossip amongst them, he should not engage in haraam transactions, and he should avoid all haraam things. If a person avoids that for a whole month then he should be able to adhere to that straight path for the rest of the year. But unfortunately many of those who fast do not differentiate between the days when they fast and the days when they do not fast, so they carry on as usual with haraam speech such as lying, cheating and so on. You do not feel that they are observing the dignity of fasting. These actions do not invalidate the fast but they do detract from its reward, and may cancel out the reward of fasting altogether.
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