From Sahih Muslim (Hadith 6059):
A'isha reported that Harith b. Hisham asked Allah's Apostle (ﷺ):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
Reference: Muslim 6059