The first form of revelation that came to the Messenger of Allah ﷺ was true dreams during sleep. He would not see a dream except that it came to pass like the breaking of dawn. The hadith continues at length.
أول ما بدئ به رسول الله ﷺ من الوحي الرؤيا الصادقة في النوم. فكان لا يرى رؤيا إلا جاءت مثل فلق الصبح… الحديث بطوله.
Bukhari narrated it in the Book of Revelation (3) and Muslim in the Book of Faith (60), both from the hadith of Ibn Shihab, who said, Urwa bin Zubayr narrated to me from Ayesha, Mother of the Believers, and he mentioned it in a long hadith, which follows.