רב דימי אחוה דרב ספרא

Rab Dimi, the brother of Rav Safra, passed away without leaving a command, and they came before Rava. It should be stated before Rabbi Abba, and he said, "I know about Rav Dimi that he was not wealthy," and he returned the deposit that was in his possession to that man (Ketubot 85b). And in Arachin [(16)], Rav Safra's brother was weak and wanted to rely on him, and he said, "Thank you that I have fulfilled all that the Rabbis say." He said to him, "Did you fulfill it?

A person should not speak well of his friend" [and he said to him, "I did not know, and if I had known, I would have fulfilled it." And in Bava Batra (94b), Rav Dimi, the brother of Rav Safra, taught, "A person should never speak well of his friend, for from his praise, etc." And it is the same in Arachin as mentioned above, except that he said, "A.D. was weak," and he said, "Thank you," etc., and he said to him, "Did you fulfill it?" etc. And in the Sefer Mitzvot Gadol, Negative Commandment 9, this Gemara is brought, and it explains that one should not speak well of his friend in front of his enemy.

See Yehuda Chayiti from the Mishnah Torah.