Is there any law in Islam that mentioned about bullying, especially verbal bullying? Or is there any fatwa concerned about that? I had looked for bullying fatwa anywhere but no result. Is it haram if I do bullying?
Bullying is prohibited straight from the Qur'an. Bullies like to call names, insult and ridicule their victims. All of these actions are prohibited in the Qur'an.
يا أيها الذين آمنوا لا يسخر قوم من قوم عسى أن يكونوا خيرا منهم ولا نساء من نساء عسى أن يكن خيرا منهن ولا تلمزوا أنفسكم ولا تنابزوا بالألقاب بئس الاسم الفسوق بعد الإيمان ومن لم يتب فأولئك هم الظالمون
O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers
They also sometimes get physical and push, hit or injure their victims. Again, this is all considered transgression and the bully is considered a wrong-doer. (The victim can retaliate against the bully, as is their right per Islam).
"Is it haram if I do bullying?" - Yes. And you can expect punishment from Allah (ﷻ) if you do so.
Here is one contemporary fatwa on the issue: Bullying in Islam