﴿ وَلَبِثُوا۟ فِى كَهْفِهِمْ ثَلَٰثَ مِا۟ئَةٍۢ سِنِينَ وَٱزْدَادُوا۟ تِسْعًۭا ﴾

“AND [some people assert], "They remained in their cave three hundred years"; and some have added nine [to that number].”

And they tarried in the Cave three hundred read with tanwīn thalāthami’atin years sinīn is an explicative supplement to thalāthami’atin ‘three hundred’ these three hundred years in the case of the People of the Cave were solar years; but for the number of lunar ones the Arabs add nine years thereto and this is mentioned in His saying and add nine that is nine years; in other words three hundred solar years while three hundred and nine lunar ones.