"The word wahy is derived from waha, meaning inspire, reveal, give an idea or impression or hasty suggestion. Wahy originally signifies the making quick sign as wahiyyun means something hasty or quick, mawtun wahiyyun means a quick death, or amrun wahyun means a fast matter. Hence, it signifies the divine words communicated to His prophets.
Wahy in the sense of "revelation" is guidance from God for His creation, brought by the Prophet, who received the words from God through one of the three modes mentioned in the Koran: "And it is not for any mortal that God should speak to him except by inspiring (wahy) or from behind a veil (min wara'i hijabin) or by sending a messenger to communicate (yuhiya) by His permission what He pleases" (42:51). It indicates three different manners of revelation:-
1. mysterious communication
2. speaking from behind the veil, and
3. the sending of a messenger
The first of these, which is called wahy is the inspiring or infusing of an idea into the heart, which is called a form of God's speaking to man. This is technically called wahy khafiyy or inner revelation.
The second mode is said to be "from behind a veil" (min wara'i hijabin) and this includes ru'ya (dream), kashf (vision) and ilham (inspiration).
The third kind, which is peculiar to the prophets, is that in which the angel brings the divine message in words, known as wahy matluww (revelation that is recited in words). It is the surest, clearest and highest form of revelation, which is limited to the prophets only. It must be noted that the effect of the angel being upon the Prophet was so great that on the coldest of days perspiration would run down his forehead in drops like pearls. (Bukhari, 1:1 and Masnad, 6:58). One tradition relates that, "froth appeared before his mouth and he roared like a young camel" (Bukhari, 1:2). Ibn Khaldun in his Muqaddimah explains this phenomenon in this way: The physical pain, he say is due to the fact that in this supernatural experience, the human soul which is not by nature prepared to experience, such a thing, is forced to leave momentarily its humanity (al-bashariyyah "human-ness") and exchanges it for angelicality (al-malakiyyah "angel-ness'), and becomes actually for the time being part of the angelic world until it resumes its human-ness.
Another Companion relates that he was sitting with his leg happening to be under the leg of the Prophet when revelation came down upon him, and the Companion felt as if his leg would be crushed under the weight (Bukhari, 8:12). Abdullah b. Amr relates, "The sura of Ma'ida was revealed to the Prophet, while he was riding on his camel. The beast could not bear him any longer, so he had to dismount from it" (Masnad, 2:176). The Koran (94:1-4) also says: "Have We not expanded for you your breast, and taken off from you your burden, which weighed down your back, and exalted your fame?"
The next point is as to the nature of the revelation. Abdullah b. Umar once asked the Prophet, how did he perceive the revelation. He replied: "Yes, I hear sound like metal being beaten" (Masnad, 2:222). On the other hand, Harith b. Hisham once enquired of the Prophet, how revelation came to him, and the reply was: "Sometimes it comes to me like the ringing of a bell (mithla salsalati al-jarasi). And this is the most hardest one me, then he leaves me and I remember from him what he says; and sometimes the angel comes in the shape of a man and he talks to me and I remember what he says." (Bukhari, 1:1)
The word jarasi (the bell) is also read as jarsi; then the phrase would mean "like some low and indistinct sound". Besides, there are several other variants: "like the noise of the beating of some metal" or "like the flopping of the wings of a bird" etc. Still, what is meant is mysterious and indefinable sound.
There are the only two forms in which the Koranic revelation came to the Prophet. In the first case, the divine words were uttered in a harsh, hard tone, which made it a heavier task for the Prophet to receive them. While in the second case the angel appeared in a human shape and uttered the words in a soft tone as one man talks to another. In both cases the Prophet was transported, as it were, to another world and this transportation caused him to go through a severe experience which made him perspired even on a cold day.