Source : Islamicity.com / 4 Mar 2013

One day, Moses delivered such an impressive sermon that all who heard it were deeply moved. Someone in the congregation asked: "O Messenger of Allah, is there another man on earth more learned than you?" Moses replied: "No!", believing so, as Allah had given him the power of miracles and honored him with the Torah.

However, Allah revealed to Moses that no man could know all there is to know, nor would one messenger alone be the custodian of all knowledge. There would always be another who knew what others did not. Moses asked Allah: "O Allah, where is this man? I would like to meet him and learn from him." He also asked for a sign to this person's identity.