Surah 18:26
"Say: ‘God knows about how long they remained (there). To Him (belongs) the unseen of the heavens and the earth. How well He sees and hears! They have no ally other than Him, and He does not associate anyone in His judgment.’"
Because this statement ends the story of the men in the cave, some scholars have suggested that this verse and verse 4 form a kind of parentheses around the story – to use the story as a warning against belief in a son of Allah. If so, the reason for the warning seems to be that belief in a son of Allah implies that Allah needs help.
- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam