The Orchas Tzadikim writes, that there was once a pious man who sent a beautiful gift to another individual. The gift was laden with all types of fruits and precious items. But the person who received the gift didn’t understand it. He wasn’t friendly with this righteous man; as a matter of fact, he was at odds with him.
So he went to the righteous man and said, “Why did you send me this elaborate gift?” The righteous man answered, “Well, I heard that you told people negative reports about me. I know that if you speak badly about me, in the Next World, they’ll take your merits and give them to me. Since you gave me a gift, I, wanted to reciprocate, so I gave you a gift.”
The Chofetz Chaim explains that when we come to the World to Come, there will be many, many merits put on our plate. Many actions that we didn’t do. Many mitzvahs that we weren’t involved in. And we’ll say, “I’d love to accept those, but they’re not mine.” And they’ll say, “Someone spoke badly about you, so we took from him and gave to you.”
The Marpeh L’Nefesh explains that this is part of HASHEM’S system of justice. When you speak badly about me, you’ve damaged me, and so there is a debt to be paid. You’ve done me a disservice, and HASHEM deems it appropriate that it be paid back. So they take merits from you and give them to me.
And so, too, the opposite. If I speak badly about you, they’ll take merits away from me and give them to you.
The Maggid Mesharim, the Maggid who appeared to the Bais Yosef many an evening, told him as follows: “When you watch people speaking lashon hara, it’s like they’re spending gold and silver.”
A man can work for years building up all types of merits, and with but a few words of speech, he gives it all away. If your tongue wags and your mouth is open, you give away more and more of your wealth. That is the impact of a tongue that’s not guarded.