From Sahih Bukhari (Hadith 4955):
Narrated Aisha:The commencement of the Divine Inspiration 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 that exists), has created man a clot. Read! And your Lord is Most Generous
Reference: Bukhari 4955