Think about this. Abraham saw a dream apparently. Thus, how would one assume it was God commanding him?
True, I think that is always very important to keep in mind. Beware IF you hear voices or see things, not beware in the sense of being scared of voices or visions, but to not follow them blind; so for "God's sake" use discrimination and Common Sense first, before acting
And it could have been God appearing to Abraham in the dream, assuming a form or a voice even, but testing Abraham
Hence I clearly understand my Master telling us "Common Sense before Divine Sense"
Even IF God (could) appear in physical form to you and tells you to do this and that THEN if it does not feel right, you better abstain.
And I think that most or all (
*) Scriptures have been inspired by God, whether it was in a dream, as a voice, in a conscious or sub/super conscious state. Knowing this, I am always very careful to accept truths given in Scriptures. And humans also easily add parts to Scriptures, because they think that they are authority. I rather stay as close to the source as possible.
For example Sai Baba:
I rather stick to His own Books, written by Him personally, than resort to Books written by His devotees, when I want the truth (best are His tapes)
For example Baha'ullah:
I trust Him more than His son for conveying something close to the truth. Even though Bahau'llah might have said His son can be trusted
For example the Koran:
I rather read Koran verses than trust interpretations given by any scholar blindly (most might be correct, but even 1 wrong one can be catastrophic)
Koran was revealed by an Angel. So it's similar than God telling Abraham in a dream (that one is even more direct, no inter medium)
According to the traditional account the Koran was revealed to Muhammad, usually by an angel, gradually over a period of years until his death in 632 C.E. It is not clear how much of the Koran had been written down by the time of Muhammad’s death, but it seems probable that there was no single manuscript in which the Prophet himself had collected all the revelations. Nonetheless, there are traditions which describe how the Prophet dictated this or that portion of the Koran to his secretaries.
* And it's a bit problematic to know for sure now, because we can't check it AND I know for a fact that humans are prone to lie and exaggerate.