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.
Dear brother in Islam, thanks a lot for your showing interest in getting acquainted with the teachings of Islam. May Allah save us all from the traps of Satan and enlighten our hearts with the light of Islam!
Islam forbids not only illicit sexual intercourse, but also forbids anything that leads to this sin and crime.
The Prophet (peace and blessings be upon him) said, “Eyes commit adultery, ears commit adultery, hands commit adultery, and feet commit adultery, and the private part either consummates it or repudiates it.” (Ahmad)
Thus, Muslims should avoid any avenue of zina (illegal sexual intercourse). Almighty Allah says, “Do not go anywhere near adultery: it is an outrage, and evil path.” (Al-Israa’ 17:32)
So Allah, in the Qur’an, not only prohibits zina, but He also prohibits even coming close to it.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Allah tells us in the Qur’an, “Do not go anywhere near adultery: it is an outrage, and evil path.” (Al-Israa’ 17:32)
So we are not only told to refrain from the actual intercourse but also forbidden to engage in any of the preliminaries that may directly or indirectly lead to fornication.
Such preliminaries include flirting, sensual conversations, and speaking of sexual matters, etc., for oftentimes one thing leads to the other. Therefore, the Prophet (peace and blessings be upon him) cautioned us against such acts saying, “Eyes commit adultery, ears commit adultery, hands commit adultery, and feet commit adultery, and the private part either consummates it or repudiates it.” (Ahmad)
Although you are certainly allowed to speak to your fiancée, your conversations at all times must be governed strictly by the Islamic ethics of interaction between males and females.
The fact that you are considering marriage with someone or that you are engaged to a person does not imply any license to break the law, for the person you are engaged to still remains a stranger to you as far as intimate relationship is concerned.
So take care not to get carried away by your chats into overstepping the limits of halal and haram.
If, as you have stated, your conversations involve sexual matters, then, I am afraid, you are overstepping the permissible boundaries.
So I advise both of you to repent sincerely to Allah and cut out such conversations cold turkey. Allah says, “Certainly, Allah loves those who are ever-repentant and those who are consistently engaged in acts of purification (through good deeds).” (Al-Baqarah 2:222)
So pray to Allah to help both of you to remain chaste, pure and modest.
Here are a few supplications (du`aa’) you may do well to recite on a regular basis:
Allahumma tahhir qalbi wa ahsin farji. (O Allah, cleanse my heart [of all traces of fornication] and help me to remain chaste and pure.)
Rabbi, qinee sharra basaree. Rabbi, qinee sharra sam`ee. Rabbi, qinee sharra lisaanee. Rabbi, qinee sharra maniyyee. (My Lord, guard me against the evils of my eyes. My Lord, guard me against the evils of my ears. My Lord, guard me against the evils of my tongue. My Lord, guard me against the evils of my private parts.)
Allahumma aghnini bi halaalika `an haraamika wa bi ta`atika `an ma`siyatika wa bi fadhlika `amman siwaaka. (O Allah, make me self-sufficient with that which You have decreed as halal for me so that I don’t need to resort to that which You have made haram for me; and let Your obedience suffice me so that I am not forced to disobey You; and let Your favor suffice me so that I am not compelled to seek favor from anyone else.)
Allah Almighty knows best.
Excerpted, with slight modifications, from: www.muslims.ca