Etiquette of Joking, Laughing, Whispering, Sitting in a Company and of Keeping the Sanctity of a Gathering:

(Tehzeeb-ul-Islam, Allama Majlisi's book)

According to a tradition coming from Imam Mohammed Baqir (a.s.), one
should not whisper when there are only three people gathered, as it is
bound to hurt the third person present.
According to the Holy Prophet (s.a.w.a.), when a person ignores and
cuts short the talk of a fellow-momin, it is like scratching the face of the
fellow-momin.
Many traditions have stated that when one sits with someone, or in
some gathering, then one should not disclose the talk which had taken
place in their company before others, unless one has taken their prior
permission.
According to the Holy Prophet (s.a.w.a.), one should keep the sanctity
of all meetings and gatherings except when murder, rape or loot were
committed.
Imam Ja'far-e-Sadiq (a.s.) states that there is no momin who is not used
to humour and is also well-behaved.
According to Imam Mohammed Baqir (a.s.), a person who enlivens
the mood of a company, is considered a friend by the Almighty God,
provided he is not using bad language for the purpose.
According to Imam Ja'far-e-Sadiq (a.s.), a momin only laughs with a
smile, not with a loud sound.
In another tradition he (a.s.) has stated that laughing loudly makes a
coward of a man and wastes faith in the same manner as salt melts in
water; and that laughing without any purpose is ignorance.
Yet in another tradition, laughing a lot has been prohibited as it spoils
the youthfulness of one's looks.
He (a.s.) has also stated that you can only joke with those you are intimate
with, but you should never use your hands and feet (i.e. no practical
jokes should be done).
A reliable tradition states that when one roars with laughter, one is
submitting oneself to shaitan and a lot of laughing robs one of one's look
of freshness.
Hazrat Ali (a.s.) has stated that one should not make fun of others as it
is understood to be a low type of abuse and it creates more improper
feelings in the other's heart.
A reliable tradition from Imam Moosa Kazim (a.s.) states that joking a
lot makes one lose the light of faith and lessens generosity and
manliness.
In another tradition it is stated that Prophet Yahya (a.s.) used to cry
only - he never smiled. Whereas Prophet Esa (Jesus) (a.s.) used to cry and
laugh as well. In another tradition it is related that Hazrat Dawood (a.s.)
asked his son Hazrat Sulaiman (a.s.) that he should not laugh a lot, for a
lot of laughing makes a man poor on the Day of Judgement.
The Holy Prophet (s.a.w.a.) has stated that for a person aware of hell it
is very difficult to laugh.
According to Imam Ja'far-e-Sadiq (a.s.), people who laugh a lot, just to
while their time or on account of corruption, will have to cry a lot on the
Day of Judgement. There are a few people who cry over their sins and
they will laugh a lot.
All these traditions emphasize that a momin should not always make a
face and be reserved with everyone; on the contrary, a momin should be
broad minded, well-mannered and laugh a little, but he should not laugh
a lot and make fun of others and it is a sigh of a low and degraded
person.