From Sahih Bukhari (Hadith 4956):
Narrated `Aisha:The commencement of (the Divine Inspirations to) Allah's Messenger (ﷺ) was in the form of true dreams. The Angel came to him and said, "Read! In the Name of your Lord Who has created all exists), has created man from a clot. Read! And your Lord is Most Generous, Who has taught (the writing) by the pen
Reference: Bukhari 4956