The Ruling on Swearing about Some Sport or Pastime

Question :

If a person swore about some sport or pastime, saying for example: "By Allah, I will not play cards as a diversion and then he played," is that considered an oath?

Answer:

Yes, an oath is whenever a person makes an oath in his heart, according to the Words of Allah, Most High:
"Allah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths."

So when a person swears by an oath intentionally, in his heart, then that is an oath. It does not matter whether it was about something, permissible, or something, obligatory, or something forbidden.

When one makes an oath, it must be examined to see if it was uttered for something good. If it was, then he should remain upon it, but if it was not, the Prophet ﷺ said:

"Whoever swore an oath, then he saw something better than it, he should atone for his oath and do the thing which is better."

So if he swore, for example, not to play cards, then he should remain with his oath and not break it, therefore he should not play cards. But if he wanted to play cards, we say to him: "Do not remain with this oath of yours; instead, abandon it and atone for your oath." And the atonement for an unfulfilled oath is to feed ten poor persons, or clothe them, or to free a slave.

And whoever is unable to do these things must fast for three consecutive days.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Pages 380-381