Hoping that he would receive the answers via wahi (divine revelation) by the next day, Rasulullah (sallallahu ‘alaihi wasallam) said, “I will give you the answers tomorrow.” However, due to his human nature, Rasulullah (sallallahu ‘alaihi wasallam) forgot to say Insha Allah (if Allah Ta‘ala wills).

After a few days passed, the verses of Surah Kahf were revealed in which the incident of the sleepers of the cave and Zul Qarnain were explained in detail. In response to the third question, the following verse was revealed, “Say (O Muhammed (sallallahu ‘alaihi wasallam))! The rooh (soul) is from the affairs of my Lord.” In other words, you will not be able to comprehend the reality of the soul. It is sufficient for you to know that the soul is the thing that when it enters the body, it comes to life, and when it departs, the body dies.”

(Extracted from Seeratul Mustafa 1/194-195)