ב"ה I have heard this idea, but I could never find a source for it. The idea is that in certain circumstances when the Torah uses the word ואמר referring to Hashem speaking that it means that the communication was specifically through the medium of dreams and not spoken word. Does anyone know the source for this?
(I can't think of an explicit connection off the top of my head, but here goes):
The Sifrei (B'midbar 153) writes that Moshe Rabbeinu's prophecy was superior to that of other prophets, because Moshe sometimes prophesied on a level connoted by the word "davar", while other prophets prophesied on a lower level as connoted by the word "amar":
The Rambam (Moreh N'vuchim 2:45) maintains that other prophets experience prophecy via dreams, whereas Moshe's prophecy involved direct communication with HaShem:
Perhaps the Rambam would specifically identify Moshe's "davar" prophecies as the direct ones.