Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
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Kissing one's wife while fasting is permissible. However, it is better to avoid kissing during the daytime of Ramadan, as it may lead to further actions which nullify the fast, such as sexual intercourse.
In his well-known book, Fiqh As-Sunnah, Sheikh Sayyed Sabiq states:
"It is confirmed that `A’ishah said: The Prophet would kiss and embrace while he was fasting, for he had the most control of all of you over his desires. 'Umar said: I was excited one time and I kissed [my wife] while I was fasting. I went to the Prophet and said: 'Today I committed a horrendous act—I kissed while I was fasting.' The Prophet asked: 'What do you think of rinsing with water while fasting?' I said: 'There is nothing wrong with that.' The Prophet said: 'Then what is the question about?'
“Ibn al-Mundhir says: ‘`Umar, Ibn `Abbas, Abu Hurairah, `A’ishah, `Ata, al-Sha`bi, al-Hassan, Ahmad, and Ishaq permit kissing. The Hanafi and Shafi`i scholars say that it is disliked if it incites one's desires. If it does not do so, it is not disliked although it is better to avoid it.’ There is no difference between an old man and a young man in this matter. The question is whether or not the kiss excites one's desires. If it does, it is disliked. If it does not, it is not disliked although it is best to avoid it. It does not matter if the kiss was on the cheek or on the lips, and so on. Touching with the hand or embracing follow the same ruling as kissing."
Elaborating on the issue, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
"In optional fasts, kissing, light touching, etc. are permitted so long as one does not engage in sexual intercourse. In obligatory fasts, one is allowed to do these only if one can control himself or herself. Breaking an obligatory fast through sexual intercourse is a serious offence, and it entails strict form of kaffaarah (expiation), which includes fasting two months consecutively; if one cannot do this, one must feed sixty poor persons for a single day of fast thus invalidated.
Therefore, no one who is fasting should do such things if there is a genuine risk of overstepping the boundaries. If there is no such fear then light touching, or kissing, etc, when not accompanied by sexual desire, is permissible. When the Prophet (peace be upon him) was asked by a young man whether he can kiss his wife while fasting, he replied no, but when an elderly person came and asked him the same question, he replied yes.
Reasons for this are clear: While it may be easier for an elderly person to control himself, that may not be the case with regard to a young person.
As far as the Prophet’s own example is concerned, his beloved wife ‘A’ishah reported that he used to kiss his wife while he was fasting. But then she added the following comment: ‘Who among you can control his sexual desire as the Prophet himself was able to control?’
In conclusion, since each person knows his own weaknesses better than anyone else, he should be his own judge in this matter and he is better advised to take necessary precautions when dealing with obligatory fasts. In case of optional fasts, however, because it does not entail any serious consequences if one were to break it because of uncontrollable sexual desire, the issue is far more flexible."
Excerpted, with slight modifications, from: www.muslims.ca