Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
quran:18:25 –

And they remained in their cave for three hundred years and exceeded by nine.  

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.
القرآن:١٨:٢٥ –

وَلَبِثُوا فِي كَهْفِهِمْ ثَلَاثَ مِائَةٍ سِنِينَ وَازْدَادُوا تِسْعًا  

{ولبثوا في كهفهم ثلاث مائة} بالتنوين {سنين} عطف بيان لثلاثمائة وهذه السنون الثلاثمائة عند أهل الكهف شمسية وتزيد القمرية عليها عند العرب تسع سنين وقد ذكرت في قوله {وازدادوا تسعا} أي تسع سنين فالثلاثمائة الشمسية: ثلاثمائة وتسع قمرية.