Here Allah tells His Messenger the length of time the people of the Cave spent in their cave, from the time when He caused them to sleep until the time when He resurrected them and caused the people of that era to find them. The length of time was three hundred plus nine years in lunar years, which is three hundred years in solar years. The difference between one hundred lunar years and one hundred solar years is three years, which is why after mentioning three hundred, Allah says, `adding nine.'
(Say: "Allah knows best how long they stayed...") `If you are asked about how long they stayed, and you have no knowledge of that and no revelation from Allah about it, then do not say anything. Rather say something like this:
(Allah knows best how long they stayed. With Him is (the knowledge of) the Unseen of the heavens and the earth.)" meaning, no one knows about that except Him, and whoever among His creatures He chooses to tell. What we have said here is the view of more than one of the scholars of Tafsir, such as Mujahid and others among the earlier and later generations.
(And they stayed in their cave three hundred years,) Qatadah said, this was the view of the People of the Book, and Allah refuted it by saying:
(Say: "Allah knows best how long they stayed...") meaning, that Allah knows better than what the people say. This was also the view of Mutarraf bin `Abdullah. However, this view is open to debate, because when the People of the Book said that they stayed in the cave for three hundred years, without the extra nine, they were referring to solar years, and if Allah was merely narrating what they had said, He would not have said,
(adding nine.) The apparent meaning of the Ayah is that Allah is stating the facts, not narrating what was said. This is the view of Ibn Jarir (may Allah have mercy on him). And Allah knows best.
(How clearly He sees, and hears (everything)!) He sees them and hears them. Ibn Jarir said, "The language used is an eloquent expression of praise." The phrase may be understood to mean, how much Allah sees of everything that exists and how much He hears of everything that is to be heard, for nothing is hidden from Him! It was narrated that Qatadah commented on this Ayah:
(How clearly He sees, and hears (everything)!) "No one hears or sees more than Allah."
(They have no protector other than Him, and He makes none to share in His decision and His rule.) meaning, He, may He be glorified, is the One Who has the power to create and to command, the One Whose ruling cannot be overturned; He has no adviser, supporter or partner, may He be exalted and hallowed.