What is the origin of the idea that whoever speaks ill of their friend carries their sins, and their merits pass to the person they spoke ill of?

To the honorable and respectable, of pure mind and ideas, the esteemed Mr. Uri Wilchfort, may he live and prosper.

Here I am to answer your questions about who speaks ill of someone, taking with them the sins of the person they spoke about, and the rights of that person passing to the one who spoke, and some details about this, as follows:

1. In the writings of the Rishonim (rabbis who lived around 800-1000 years ago), it is explained that whoever speaks ill of their neighbor loses their merits and transfers them to the person they slandered, and also their transgressions are transferred to the one who spoke ill of them, but it is not clear in their words that this refers to all merits and all transgressions. And, in my opinion, it seemed that the intention was only for all the sins and crimes of slander, or only for that sin about which he spoke ill of his neighbor, and thus it is understood from the words of Chovot HaLevavot. However, our sages acharonim of blessed memory (rabbis from 500 years ago to the contemporary era) clearly described that the intention is for all commandments and transgressions, due to the severity of the sin of slander compared to all the transgressions of the Torah.
2. He who spoke ill of him will not escape all his sins without repentance, but when they spoke ill of him and he remained silent and accepted the divine judgment, his repentance is light and the sin of the speaker rises to the heavens, proportionally to the sins of the one who was spoken about. And even those who are stoned, hanged, and other executions by the rabbinical court have no forgiveness except through repentance, and Yom Kippur does not forgive except for those who repent.
3. All the severity of this matter specifically applies to the slanderer, that is, one who is always accustomed to spreading rumors and speaking ill of others, but does not refer to a person who occasionally made the mistake of speaking ill.

Regarding the slanderer, who has taken on the sins of others about whom he spoke, and lost his merits, his penance is difficult, for he is accustomed to speaking ill and finds it hard to turn away from this evil path. However, if he masters his desire and does penance, the Holy One, Blessed Be He, does not take revenge on His creatures, and his penance is accepted and welcomed before the King who sits on the throne of mercy, and all his sins are forgiven, and his merits are restored. And there are those who say that also his neighbor, about whom he spoke ill, will receive a reward corresponding to the merits he has already obtained, and will not lose with his friend’s penance and reconciliation, for it has already been granted to him from heaven.

With greetings

Rabbi Yosef Chay Siman Tov