Harmful is all Harsh Speech, which grows only Conflict & Hostility!
Once in
Savatthi the Blessed
Buddha said this:
What, householder friends, is the Dhamma explanation befitting for oneself?
Here, householder friends, any Noble Disciple reflects thus: If someone were
to
to address me with harsh speech, abusive, aggressive, offensive, ruthless &
bitter,
that would neither be pleasing nor agreeable to me. Similarly, if I were to
address
another with harsh speech, abusive, aggressive, offensive, ruthless & bitter,
that
would neither be pleasing, nor agreeable, nor acceptable to that other being
either!
What is displeasing and disagreeable to me, is also displeasing and
disagreeable
to any other being too. How can I harm & hurt another being with what provoke
annoy and exasperate myself? Having reflected repeatedly thus, then
gradually:
1: He/she will carefully avoid all harsh, abusive, aggressive, & offensive
speech...
2: He/she will persuade others also to abstain from all insulting & malign
speech...
3: He/she will praise speaking only kind and gentle words worth
remembering...
In this very way, is this advantageous verbal behaviour purified in 3
aspects!!
Buddha once noted:
Beings are born with an axe in their mouth by which they cut their own
future down...
Source (edited extract):
The Grouped Sayings of the Buddha.
Samyutta Nikāya. [V:355]
section 55:
Sotāpattisamyutta.
Thread 7:
To the people at the Bamboo gate...
One should
never harm other beings by Hurting them with Harsh Speech!

More
on not whipping by speech, not stabbing with dagger-like words:
https://What-Buddha-Said.net/drops/Friendliness_Frees.htm
https://What-Buddha-Said.net/drops/II/Anger_and_Irritation.htm
And on Right Speech (Sammā Vācā):
https://www.accesstoinsight.org/ptf/samma-vaca.html
https://What-Buddha-Said.net/drops/Truth_Triumphs.htm
https://What-Buddha-Said.net/drops/IV/Not_Splitting.htm
https://What-Buddha-Said.net/drops/IV/Not_Scolding.htm
https://What-Buddha-Said.net/drops/IV/Not_Deceiving.htm
https://What-Buddha-Said.net/drops/IV/Not_Gossiping.htm
https://What-Buddha-Said.net/drops/II/The_Three_Doors.htm
https://What-Buddha-Said.net/drops/What_is_Right_Speech.htm
https://What-Buddha-Said.net/drops/IV/Fourfold_Right_Speech.htm
https://What-Buddha-Said.net/drops/III/Optimal_Observance_IV.htm
Avoid all Harsh & Aggressive
Speech!