Regardless of Whether You are Serious or Joking, Lying is Forbidden

Question :

When some people joke with their friends, they say false things to get each other to laugh. Isn't this wrong in Islam?

Answer:

Yes, this is a sin in Islam, because all lying is a sin which must be avoided. The Messenger of Allah ﷺ said:
"Be truthful, for truthfulness guides to righteousness, and right- eousness leads to Paradise. A person continues to be truthful and continues to be a seeker of truth until he is written, with Allah, as a truthful one. Beware of lying, for lying guides to wickedness, and wickedness leads to the Fire. A person continues to lie and find ways of lying until he is written, with Allah, as a liar."

On another occasion, the Prophet ﷺ said:
"Woe to him who tells lies so that others may laugh woe to him, and again, woe to him."

Based upon this, lying must be avoided whether it is for the sake of laughter, in seriousness or in jest. A person who trains himself to be truthful until he makes it his habit becomes truthful both outwardly and inwardly, which is why the Messenger of Allah ﷺ said:
"A person continues to be truthful and continues to be a seeker of truth until he is written, with Allah, as a truthful one."

None of us, I am sure, is ignorant of the results of truthfulness and the results of lying.

Source:
Ash-Shaykh Ibn Baz
Fatawa Islamiyah, Vol. 8 Pages 290-291