Sahih Al Bukhari 1: Chapter 1, Hadith 2

Narrated 'Aisha: (the mother of the faithful believers) Al-Harith bin Hisham asked Allah's Apostle O Allah's Apostle! How is the Divine Inspiration revealed to you? Allah's Apostle 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 being inspired divinely on a very cold day and noticed the sweat dropping from his forehead (as the Inspiration was over).

 حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ‏‏‏‏‏‏ٍ‏‏‏‏، قَالَ:‏‏‏‏ أَخْبَرَنَا مَالِكٌ‏‏‏‏‏‏ٍ‏‏‏‏، عَنْ هِشَامِ بْنِ عُرْوَةَ‏‏‏‏‏‏ٍ‏‏‏‏، عَنْ أَبِيهِ‏‏‏‏‏‏ٍ‏‏‏‏، عَنْ عَائِشَةَ‏‏‏‏‏‏ٍ‏‏‏‏أُمِّ الْمُؤْمِنِينَ رَضِيَ اللَّهُ عَنْهَا، ‏‏‏‏‏‏أَنَّ الْحَارِثَ بْنَ هِشَامٍ رَضِيَ اللَّهُ عَنْهُ سَأَل رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏فَقَالَ:‏‏‏‏ يَا رَسُولَ اللَّهِ، ‏‏‏‏‏‏كَيْفَ يَأْتِيكَ الْوَحْيُ؟ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ:‏‏‏‏ "أَحْيَانًا يَأْتِينِي مِثْلَ صَلْصَلَةِ الْجَرَسِ، ‏‏‏‏‏‏وَهُوَ أَشَدُّهُ عَلَيَّ فَيُفْصَمُ عَنِّي، ‏‏‏‏‏‏وَقَدْ وَعَيْتُ عَنْهُ مَا قَالَ، ‏‏‏‏‏‏وَأَحْيَانًا يَتَمَثَّلُ لِي الْمَلَكُ رَجُلًا فَيُكَلِّمُنِي فَأَعِي مَا يَقُولُ"، ‏‏‏‏‏‏قَالَتْ عَائِشَةُ رَضِيَ اللَّهُ عَنْهَا:‏‏‏‏ وَلَقَدْ رَأَيْتُهُ يَنْزِلُ عَلَيْهِ الْوَحْيُ فِي الْيَوْمِ الشَّدِيدِ الْبَرْدِ فَيَفْصِمُ عَنْهُ، ‏‏‏‏‏‏وَإِنَّ جَبِينَهُ لَيَتَفَصَّدُ عَرَقًا.

REFERENCE: Sahih Al Bukhari 1: Chapter 1, Hadith 2