He Swore Not to do Something, Then He Did it Forgetfully

Question :

The Following letter was received by the Committee:
A man swore by Allah that he would not shake hands with women. After some time, he entered a meeting in which there were women among the neighbors present, and he shook hands with them, forgetting his previous oath. Now he is asking what is the consequence of it.

Answer:

Having studied the request for a legal verdict, the Committee answered as follows:

If the matter is as stated by the questioner, that he shook hands with women after swearing an oath that he would not shake hands with them, and that he did so due to forgetfulness, then there is no sin upon him, according to the Words of Allah, Most High:
"Our Lord! Punish us not if we forget or fall into error."

And it has been authentically reported from the Prophet that he said:
"My people have been pardoned for their mistakes, their forgetfulness and the things which they are forced to do."

If he does something in the future (ie, shaking hands with women) when he is aware of it, and he does it deliberately, he must make the atonement for an unfulfilled oath. He must also bear in mind that it is not permissible for him, according to the Islamic Law, to shake hands with women, unless they are Mahrams for him, such as his mother, his sister, his daughter and their like.

And may peace and blessings of Allah be upon our Prophet Muhammad ﷺ.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Pages 374-375