It is forbidden to tell a person a remark which was made about him, if that remark will cause him to be even slightly upset — though it contains nothing which is actually derogatory. Relating such remarks falls under the category of avak rechilus.
As proof of this law, the Chofetz Chaim (in Be’er Mayim Chaim) cites the famous incident involving Avraham and Sarah (cited above in Day 92). When the angels, who were disguised as wayfarers, informed Avraham that in a year hence he and his wife would be blessed with a child, Sarah (who stood listening at the doorway of the tent) laughed incredulously. “After I have withered shall I again have delicate skin? And my husband [too] is old!” (Bereishis 18:12). Hashem was displeased with this response (see Ramban ad loc.) and demanded of Avraham, “Why is it that Sarah laughed, saying, ‘Shall I in truth bear a child though I have aged?’” For the sake of peace between husband and wife, Hashem did not tell Avraham that Sarah had mentioned that he, too, was old.
Let us ponder this for a moment. At the time of this incident, Avraham was ninety-nine years old. Sarah was certainly not saying something derogatory when she referred to her husband as “old.” But the relationship between husband and wife is a very delicate one. Hashem, in His infinite wisdom, determined that even someone as great as Avraham Avinu (our Forefather) might feel slightly hurt upon learning that his wife referred to him as “old.” Therefore, Hashem omitted this part of Sarah’s remark when confronting Avraham.
The Chofetz Chaim concludes this chapter by cautioning us not to reveal private information which has been told to us in confidence. While revealing secrets is not necessarily in the category of speaking rechilus, it does cause harm to the person who confided in us. Furthermore, says the Chofetz Chaim, one who reveals secrets steps beyond the bounds of tznius (privacy and modesty) and goes against the wishes of the other person, which in itself is wrong.