Do not mention the defects of others

Narrated by Mu'adh bin Jabal, radhiAllaahu 'anhu: Allaah's Messenger, sallAllaahu 'alayhi wa sallam, said,

"Whoever insults his brother due to a sin (which he has committed), will not die until he commits it himself."

[At-Tirmidhi reported it and graded it hasan (good), while its chain is Munqati' (broken)]

Footnotes: We thus know that there mere mention of the defects of another person is a sinful act. It is even worse to mention someone's shortcomings in front of others, with the intention of putting him to shame. A Hadith states that one should not remind anyone of his sin in reply to something he has said (which you may not like).

Bulugh al-Maram