How did the Messenger, may Allah’s prayers and peace be upon him, see the people of Paradise and the people of Hell during the Isra and Mi'raj before the reckoning, and is this among the undisputed matters of the unseen?

The incident of the Isra’ (Night Journey) is one of the great miracles of the Prophet, peace and blessings be upon him. In it, Allah revealed to him Paradise and Hell. The Prophet, peace and blessings be upon him, also saw this in his dream and on the day the sun was eclipsed in Medina. These observations are among the unseen matters in which one must believe. And Allah Almighty is the One who revealed to him the destiny of some people in the barzakh (interim period between death and resurrection).