Chabad.org — English translation + Rashi
Torah Temimah points to Yoma 4b (same amud; focus on leimor).
E-Daf.com — Yoma 4b
HebrewBooks / Shas PDF
English: “Saying” (leimor) — Rabbi Musya grandson of Rabbi Masya said in the name of Rabbi Musya the Great: From where do we know that one who says something to his fellow is bound not to repeat it further until [the original speaker] says to him, “Go, say [it]”? As it is stated: “And Hashem spoke to him from the Tent of Meeting, saying.”
Restrictive reading of leimor — private speech may not be repeated without explicit leave to transmit.
What is told to you in confidence is not yours to retell. The default is silence toward third parties until the original speaker authorizes transmission (“go, say”). The derasha is hung on the opening of Vayikra so that even sacred communication is modeled with controlled release, not automatic republication.