How did Prophet Muhammad receive revelation?
The following paragraphs are from the book The Quran: An Eternal Challenge by a very learned scholar, Muhammad Abdullah Draz.
We have all heard of the remarkable physical changes that were apparent on the Prophet’s face whenever he received a Quranic revelation from on high. Those who were present would see his face turn red and he would feel so hot that sweat drops would appear on his forehead. He would also become heavy to the extent that his thigh would press hard against the thigh of the person next to him. If he was riding, his mount would lay down. At the same time, noises like the buzzing of bees would be heard near his face (these physical experiences are confirmed in an authentic hadith related by Bukhari, Muslim and others). These manifestations would eventually subside and the Prophet (peace and blessings be upon him) would relax and recite the Quranic verses he had received.
It is from this point of departure that we should proceed to investigate the physical effects of revelation on the Prophet (peace and blessings be upon him) because it is here that truth will be determined. When we have a causal relationship between two different things wherein one is the direct consequence of the other, a proper investigation of the relationship will demonstrate the true cause of the effect.
Let us now consider the phenomenon: Was it conscious and deliberate even as a preparatory means to achieve a higher state of concentration? Or was it entirely involuntary? If so, was it induced by normal conditions such as lethargy or drowsiness or an abnormal condition such as a mental disturbance? Or was it a reaction to something totally external?
A glance at the components of the phenomenon suffices to show that the Prophet’s symptoms could never have been the result of affectation. This becomes even more apparent when we consider the noises that could be heard near his face when he received revelation. Moreover, if the Prophet (peace and blessings be upon him) did indeed affect these physical experiences, he would have been able to resort to them at any time and produce any verse at will. However, we know that on many occasions the Prophet (peace and blessings be upon him) did not receive revelation when he desperately needed it but only when God willed. Hence, it was clearly involuntary.
If we examine the matter deeper, we will find that the symptoms are in no way similar to the drowsiness we experience when we need to sleep. The Prophet (peace and blessings be upon him) experienced this condition in all situations, when he was seated, standing, walking or riding, morning and evening, and even when he was talking to others, whether friends or foes. Revelation would come unexpectedly and cease just as suddenly. It would last only for a very brief period. It was not gradual like lethargy. It was always accompanied by strange noises that did not emanate from him or from anyone else when they go to sleep. This was a condition that lacked any of the features of sleep.
We also find it totally unlike any symptom of mental illness or fits and seizures, for these are normally accompanied by a pallid complexion, cold extremities, teeth chattering, involuntary exposure of body parts, and blackouts. The difference, as we have seen earlier, is that Muhammad's experience involved increased physical strength, a vibrant complexion, increased body warmth and resulted in light and knowledge. In fact, every piece of revelation produced compelling wisdom that is readily accepted by the human mind.