What should I do to erase what I said when I described my brother as a failure and mocked him, after I learned that Sheikh Al-Islam Ibn Taymiyyah said: "Whoever taunts someone for a sin will not die until he commits it," and fearing that I will graduate with the same grade? And do I have to apologize to my brother, knowing that he did not hear my abusive words?

Scoffing at and ridiculing others is not permissible, due to Allah's saying: ﴿O YOU WHO HAVE BELIEVED, LET NOT A PEOPLE RIDICULE [ANOTHER] PEOPLE﴾. It is a form of arrogance, which is one of the major sins.

If you regret your action, apologize to your brother and seek his forgiveness by saying: "I was wrong towards you, so please forgive me," without mentioning that you scoffed at him. If doing so would lead to harm, then supplicate for him abundantly in his absence and speak well of him in the gatherings where you scoffed at him.

As for what was mentioned about Ibn Taymiyyah, it could not be found. However, in Al-Harawi's "Manazil as-Sa'irin," it states: "Every sin for which you have reproached your brother will return to you." Ibn al-Qayyim commented on this, saying that it will revert to you, and he cited the hadith: "Whoever reproaches his brother for a sin will not die until he commits it," which was graded as weak by Al-Albani.