Hadith 

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ ـ رضى الله عنها ـ أَنَّ الْحَارِثَ بْنَ هِشَامٍ ـ رضى الله عنه ـ سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ كَيْفَ يَأْتِيكَ الْوَحْىُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَحْيَانًا يَأْتِينِي مِثْلَ صَلْصَلَةِ الْجَرَسِ ـ وَهُوَ أَشَدُّهُ عَلَىَّ ـ فَيُفْصَمُ عَنِّي وَقَدْ وَعَيْتُ عَنْهُ مَا قَالَ، وَأَحْيَانًا يَتَمَثَّلُ لِيَ الْمَلَكُ رَجُلاً فَيُكَلِّمُنِي فَأَعِي مَا يَقُولُ ‏"‏‏.‏ قَالَتْ عَائِشَةُ رضى الله عنها وَلَقَدْ رَأَيْتُهُ يَنْزِلُ عَلَيْهِ الْوَحْىُ فِي الْيَوْمِ الشَّدِيدِ الْبَرْدِ، فَيَفْصِمُ عَنْهُ وَإِنَّ جَبِينَهُ لَيَتَفَصَّدُ عَرَقًا‏.‏ 

Narrated 'Aisha: 

(the mother of the faithful believers) Al-Harith bin Hisham asked Allah's Messenger (ﷺ) "O Allah's Messenger (ﷺ) ! How is the Divine Inspiration revealed to you?" Allah's Messenger (ﷺ) replied, "Sometimes it is (revealed) like the ringing of a bell, this form of Inspiration is the hardest of all and then this state passes off after I have grasped what is inspired. Sometimes the Angel comes in the form of a man and talks to me and I grasp whatever he says." 'Aisha added: Verily I saw the Prophet (ﷺ) being inspired divinely on a very cold day and noticed the sweat dropping from his forehead (as the Inspiration was over). 

Sahih 

Sahih al-Bukhari, 2

In-Book Reference: Book 1, Hadith 2

USC-MSA web (English) reference: Vol. 1, Book 1, Hadith 2 (deprecated numbering scheme)