These two people went to Madinah Munawwarah and informed the Jewish scholars of what was happening in Makkah Mukarramah. The Jews suggested that they pose three questions to Rasulullah (sallallahu ‘alaihi wasallam):

The first question was, “Who were the people who sought refuge in a cave and what is their incident?” In other words, they would ask Rasulullah (sallallahu ‘alaihi wasallam) regarding the incident of the people of the cave.

The second question was, “Who was the man who traversed the entire earth from the East to the West?” In other words, they would ask Rasulullah (sallallahu ‘alaihi wasallam) regarding the incident of Zul Qarnain.

The third question was, “What is the rooh (soul)?”

The Jewish scholars further mentioned that if Rasulullah (sallallahu ‘alaihi wasallam) answered the first two questions and remained silent regarding the third question, then they should understand that he is a true Nabi, otherwise he is a liar and impostor.”

(Extracted from Seeratul Mustafa 1/194)