Sahih Muslim | Book 30 | Hadith Number 5765

Narrated by 'Aisha
'Aisha reported that Harith b. Hisham asked Allah's Apostle (may peace be upon him): 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.