Sayyidi wa sanadi Mufti Mohammad Taqi Usmani (Allah preserve him) said,
The Messenger of Allah (Allah bless him and give him peace) said: “That person who taunts and ridicules his Muslim brother over a sin from which he has repented, will not die until he himself commits that same sin.” For example, you come to know that a certain person committed or was involved in a particular sin and you also know that this person has repented from it. To think low of him or to taunt or ridicule him because of that sin, by saying something like: “You are the one who was involved in certain evil actions”, is in itself a sin.
Through repentance a person has corrected his relationship with Allah Most High. Through repentance not only has the sin been forgiven, it has also been erased from his book of deeds! Allah Most High has erased it from his book of deeds but you, because of that sin, are thinking low of him and treating him with contempt. You are taunting and ridiculing him. This action is extremely despised by Allah Most High.
Source: Deoband.org