Cave of Ashabe-Kahf (interior)

The view above shows a section of the interior of the Ashabe-Kahf cave. The crafted stone blocks are tombs; one of them (left) has a viewing hole through which bones can be seen. The bones of the dog have also been displayed.

• The Prophet (ﷺ) was challenged by the Quraysh of Makkah who did not believe in his message to answer three questions passed onto them by the Jews. One of the questions was, “Who were the youth who disappeared, and how many were they?” The Jews knew that the Prophet (ﷺ) would only be able to tell the story if he was indeed a prophet.
• The Prophet (ﷺ) had no clue and said that he would answer them tomorrow, hoping for the answer to be revealed to him through Jibraeel (عليه السلام) but forgetting to say ’Inshallah’ (if Allah wills). No revelation came to the Prophet (ﷺ) for 15 days, leading the Quraysh to greatly abuse and revile him.

• The answer, however, was eventually revealed to the Prophet (ﷺ) in a Surah named after the cave (Al-Kahf) of the sleepers. The Quran revealed the exact story that the Jews knew of, and it answered the questions similarly to the information they had. The Quran confirmed that they slept for 309 years, which the Jews knew of. The Quran however did not give an exact answer as to how many they were. Allah (ﷻ) revealed: “[Some] say, ‘The sleepers were three, and their dog made four,’ others say, ’They were five, and the dog made six’-  guessing in the dark – and some say,’ They were seven, and their dog made eight.’ Say [Prophet], ‘My Lord knows best how many they were.’ Only a few have real knowledge about them, so do not argue, but stick to what is clear, and do not ask any of these people about them.” [18:22]

• The Jews themselves did not know exactly how many there were (whether 3,5 or 7) and were astonished when the Quran gave all the possible numbers they had suspected for the number of sleepers.

References:  Stories of the Prophets – Ibn Kathir, Wikipedia