CLAIM: The author of Hebrews writes of “the word spoken through angels” (Heb. 2:2). What does he mean by this and when did this happen?
RESPONSE: There is only one passage about this in the OT.
(Deut. 33:2) He said, “The Lord came from Sinai, and dawned on them from Seir; He shone forth from Mount Paran, and He came from the midst of ten thousand holy ones; at His right hand there was flashing lightning for them.”
There are a number of passages in the NT that mention this event:
(Acts 7:53) “You who received the law as ordained by angels, and yet did not keep it.”
(Gal. 3:19) Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made.
There is also mention of this event in the Septuagint, Josephus, and early rabbis. Leon Morris writes, “In the LXX of Deuteronomy 33:2, and in Josephus (Antiq. XV, 136). The rabbis also thought of angels as there on that great occasion (SBK, 3:554–56).”
 Morris, L. (1981). Hebrews. In F. E. Gaebelein (Ed.), The Expositor’s Bible Commentary, Volume 12: Hebrews through Revelation (F. E. Gaebelein, Ed.) (21). Grand Rapids, MI: Zondervan Publishing House.