You can read more about the Seven Sleepers at Wikipedia; this is like a "Rip van Winkle" story, and it is a famous legend in both the Christian and Muslim tradition.
THE SEVEN SLEEPERS
Now, as to the Seven Sleepers, these were seven young Christians who lived in the time of the Emperor Decius. They were Maximian, Malchus, Marcus, Dionysius, John, Serapion, and Constantine. They were accused before the ruler because they refused to offer sacrifices to the gods, but they escaped to Mount Coelion [near Ephesus] and hid in a cavern. They were, however, betrayed and discovered as they slept, and by order of the Emperor were sealed into the cave.
But two Christian men, Theodorus and Rufinus, wrote an account of their martyrdom and laid the parchment, closed with silver seals, among the stones that blocked the cave's mouth.
Two hundred and eight years later, when the Christian Emperor Theodosius was reigning, he sorrowed over the heresy which had sprung up, denying the resurrection of the dead. Daily he retired to a secret place in his palace and mourned in sackcloth and ashes. God in the following manner rewarded and justified his faith.
A certain burgess of Ephesus, in digging to make a stable on Mount Coelion, came upon the cavern; the light and air entered it, and the sleepers awoke, thinking that they had slept but over night.
One of their number, Malchus, was selected to go to the city to buy food. He set forth with fear and trembling, supposing his life and that of his companions to be in great danger.
He advanced cautiously toward the city gate and was startled at the sight of a cross placed above it. He tried another gate, there found another cross. He believed himself to be dreaming, but covering his face he entered the city and went to buy bread. Hearing everywhere the name of Christ spoken boldly and openly he was more and more puzzled.
Finally he arrived at a baker's and bought bread. In payment he offered a coin the like of which the baker had never seen; the baker therefore came to the conclusion that Malthus had found some ancient hidden treasure; he immediately accused him, and Malchus knew not what to say for dread. When the bystanders saw his confusion they put a cord round his neck and haled him before the consul and the bishop.
After much questioning, the truth was discovered, and Malchus led all the principal men of the city, followed by the multitude, to the cave in Mount Coelion. There within sat the other youths, "their visages like unto roses flowering" and shining like the sun. Also they found the letter of Theodorus and Rufinus, sealed with its seals of silver, among the displaced stones at the mouth of the cave.
The Emperor was sent for and came in haste. Entering the cave, he with tears embraced each of the seven, saying: "I see you as if I should see our Lord raising Lazarus!"
Maximian then said to him: "Believe us, O Emperor, for our Lord has raised us before the Day of Judgment to the end that thou believe firmly in the resurrection of the dead!"
After these words, they all inclined their heads to the earth, and rendered up their spirits to God.
The Emperor wished to make sepulchres of gold and silver for them, but they appeared to him that night and bade him suffer them to remain on the earth of the cave (on their right side), as they had so long lain, until the day of the final resurrection.
Here is a Russian icon that depicts the sleepers: