I had read some notes that I had jot down in a mosque about this figure or might be from a reading somewhere. I can't really remember about the source.
This person whose name is Umais al-Qarni was a follower of prophet Muhammad (saw) who lives in the time of the prophet and throughout his life, he had never met the prophet (saw) because he has to take care of his mother while he had for long received the message of Islam.
He was longing to see prophet Muhammad (saw) until one day, he asked the permission of his mother. His mother had let him off to see the prophet (saw) but he must come back as soon as possible. When he arrived there in Medina, he finds out that the prophet (saw) was not there because he was in a battle. So, disappointed he was and he went back to Yemen.
Umais al-Qarni was just a poor shephard, and he had finished all of his money for the expenses to travel to Medina to see prophet Muhammad (saw). He had lost all of his hope to see prophet Muhammad (saw).
When the prophet Muhammad (saw) reached home, his wife which is our lady Aisha (r.a) wanted to tell him about the guest. But the prophet (saw) had first had this question popped out of him, "somebody wanted to see me wasn't he? And he has white mark on his hand. Allah had first informed the prophet (saw) about Umais.
Later, prophet Muhammad (saw) told his companions, "if all of you meet this person whose name is Umais al-Qarni, ask him to pray for you and ask him to ask forgiveness from Allah for you since he is the inhabitant of the heaven."
Some people questioned whether this person by the name Umais al-Qarni is considered as a companion of prophet Muhammad (saw) or not because a companion of prophet (saw) must be someone who had met the prophet (saw) in his lifetime.
During the administration of Umar al-Khattab, the caliph wanted to see Umais al-Qarni. So, he went to Yemen and he met this person with white spot on his hand. He asked Umais to pray for him, but Umais wondered. He is just an illiterate poor shephard and why is this caliph asking him to do so?
From this story that our teachers had told us, Allah had placed Umais in such an honor because of his love toward prophet Muhammad (saw). In the same time, he is also a loyal and a caring child who takes care of his mother.
There is also an Arabic proverb to describe this event of Umais, which is translated as human are always enslaved by their circumstances.