In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Aa’isha [radiyallaahu anhaa] said: Al-Haarith bin Hisham asked Allaah’s Messenger [sallal laahu alayhi wasallam]: “O Allaah’s Messenger! How is the Divine Revelation revealed to you?” Allaah’s Messenger  replied, “Sometimes it is (revealed) like the ringing of a bell, this form of Revelation is the hardest of all and then this state passes off after I have grasped what is revealed. Sometimes the angel comes in the form of a man and talks to me and I grasp whatever he says.” Aishah added: Verily I saw the Prophet being inspired [divinely] and [noticed] the sweat dropping from his forehead on a very cold day as the Revelation was over. [Al-Bukhaari’ Number’ Hadeeth Number 2]
Some Points Derived From This Hadeeth
1. The heaviness of the revelation on the Messenger [sallal laahu alayhi wasallam], and this is affirmed by one of the explanations of the Aayah [i.e. by the scholars of Tafseer]:

Verily, We shall send down to you a weighty Word.”

[Surah Al-Muzzammil’ Aayah 5]
2. The Sahaabah witnessed the [physical] state of the Messenger [sallal laahu alayhi wasallam] whilst the revelation was been sent down to him.
3. The statement of Aa’isha [radiyallaahu anhaa]: “Verily I saw the Prophet being inspired [divinely] and [noticed] the sweat dropping from his forehead on a very cold day”: [This shows that some of the] Qur’aan was revealed in winter and summer, as stated in a hadeeth [reported by Umar who said that the Messenger said to him]: O Umar! “Does this verse that is at the end of Surah An-Nisaa’, which was revealed in summer not suffice you? [i.e. regarding Al kalaalah – those who leave neither descendants nor ascendants as heirs] [Saheeh Muslim’ Number 567]
4. This hadeeth contains a refutation against the one [i.e. some of the orientalists, enemies of Islaam etc] who say that the revelation received by him [sallal laahu alayhi wasallam] was as a result of epileptic fits, because the Sahaabah witnessed the [physical state of the Prophet] and did not see any of this [i.e. this false claim of these enemies of Islaam].
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Source: An Excerpt from ‘Al-Arba’oona Hadeethaa Fee Uloomil Qur’aan. pages 9-13. By Shaikh Muhammad Baazmool [hafidhahullaah]. slightly paraphrased