The Ruling on Relations Before Marriage

Question :

What is the ruling of the relationship of a person with his Fiancée?

Answer:

If what is meant by the saying of the questioner "before marriage" before consummating it, and after the contract is made, then there is no sin, because after the contract is made, she will be his wife, even if it has not been consummated. However, if it was before the contract, and during the proposal or before it, then it is forbidden, impermissible, because it is not allowed for a man to enjoy the company of a woman who is unrelated to him, neither by speech, nor by looking at her, nor by being alone with her.

This is because it has been confirmed from the Prophet SAW that he said:
"No man should stay with a lady in seclusion except in the presence of a Dhū-Mahram. A woman should not travel except with a Dhū-Mahram"

In short, it may be said that there is no sin in meeting after the contract has been made. But if it is before the contract, even if it is after the proposal and her acceptance, it is not permissible, and he is forbidden to her because she is not related to him until he makes the contract with her.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 5 Pages 122-123