When revelation descended upon Messenger of Allah ﷺ even during the cold days, his forehead perspired.  

إِنْ كَانَ لَيُنْزَلُ عَلَى رَسُولِ اللَّهِ ﷺ فِي الْغَدَاةِ الْبَارِدَةِ ثُمَّ تَفِيضُ جَبْهَتُهُ عَرَقًا  

How does the the wahi (inspiration) come to you? He said: At times it comes to me like the ringing of a bell and that is most severe for me and when it is over I retain that (what I had received in the form of wahi), and at times an Angel in the form of a human being comes to me (and speaks) and I retain whatever he speaks.  

أَنَّ الْحَارِثَ بْنَ هِشَامٍ سَأَلَ النَّبِيَّ ﷺ كَيْفَ يَأْتِيكَ الْوَحْىُ فَقَالَ أَحْيَانًا يَأْتِينِي فِي مِثْلِ صَلْصَلَةِ الْجَرَسِ وَهُوَ أَشَدُّهُ عَلَىَّ ثُمَّ يَفْصِمُ عَنِّي وَقَدْ وَعَيْتُهُ وَأَحْيَانًا مَلَكٌ فِي مِثْلِ صُورَةِ الرَّجُلِ فَأَعِي مَا يَقُولُ  

'Ubida b. Samit reported that when wahi (inspiration) descended upon Messenger of Allah ﷺ, he felt a burden on that account and the colour of his face underwent a change.  

كَانَ نَبِيُّ اللَّهِ ﷺ إِذَا أُنْزِلَ عَلَيْهِ الْوَحْىُ كُرِبَ لِذَلِكَ وَتَرَبَّدَ وَجْهُهُ  

'Ubida b. Samit reported that when wahi descended upon Allah's Apostle ﷺ, he lowered his head and so lowered his Companions their heads, and when (this state) was over, he raised his head.  

كَانَ النَّبِيُّ ﷺ إِذَا أُنْزِلَ عَلَيْهِ الْوَحْىُ نَكَسَ رَأْسَهُ وَنَكَسَ أَصْحَابُهُ رُءُوسَهُمْ فَلَمَّا أُتْلِيَ عَنْهُ رَفَعَ رَأْسَهُ