How the Revelation comes down
Allah says,
وما كان لبشر أن يكلمه الله إﻻ وحيا
It is not given to any human being that Allah should speak to him unless (it be) by revelation,
This refers to how Allah sends revelation. Sometimes He casts something into the heart of the Prophet, and he has no doubt that it is from Allah, as it was reported in Sahih Ibn Hibban that the Messenger of Allah said:
إن روح القدس نفث في روعي أن نفسا لن تموت حتى تستكمل رزقها وأجلها فاتقوا الله وأجملوا في الطلب
Ar-Ruh Al-Qudus (i.e., Jibril) breathed into my heart that no soul will die until its allotted provision and time have expired, so have Taqwa of Allah and keep seeking in a good (and lawful) way.
أو من وراء حجاب
or from behind a veil --
as He spoke to Musa, peace be upon him. He asked to see Him after He had spoken to him, but this was not granted to him.
In the Sahih, it recorded that the Messenger of Allah said to Jabir bin Abdullah, may Allah be pleased with him:
ما كلم الله أحدا إﻻ من وراء حجاب وإنه كلم أباك كفاحا
Allah never speaks to anyone except from behind a veil, but He spoke to your father directly.
This is how it was stated in the Hadith.
He (Jabir's father) was killed on the day of Uhud, but this refers to the realm of Al-Barzakh, whereas the Ayah speaks of this earthly realm.
أو يرسل رسوﻻ فيوحي بٳذنه ما يشاء
or (that) He sends a Messenger to reveal what He wills by His leave.
as Jibril, peace be upon him, and other angels came down to the Prophets, peace be upon them.
إنه علي حكيم
Verily, He is Most High, Most Wise.
He is Most High, All-Knowing, Most Wise