'A'isha reported: When revelation descended upon Allah's Messenger (ﷺ) even during the cold days, his forehead perspired.
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'A'isha reported that Harith bin Hisham asked Allah's Apostle (ﷺ) : How does 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.
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Explanation & Benefits
Shaykh Maulana Abdul Aziz Alvi
Hadith Commentary: Benefits and Issues: Since you (sallallahu alayhi wa sallam) had to endure great hardship at the arrival of revelation (wahy), even in cold weather your forehead would become drenched with sweat.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 6058