The first form in which revelation came to the Messenger of Allah ﷺ was true dreams during sleep. He would never see a dream except that it came to pass as clearly as the break of dawn.
أول ما بدئ به رسول الله ﷺ مر الوحي الرؤيا الصادقة في النوم النوم، فكان لا يرى رؤيا إلا جاءت مثل فلق الصبح.