Surah Al-Kahf 18:60

وَإِذْ قَالَ مُوسَىٰ لِفَتَىٰهُ لَآ أَبْرَحُ حَتَّىٰٓ أَبْلُغَ مَجْمَعَ ٱلْبَحْرَيْنِ أَوْ أَمْضِىَ حُقُبًا

And [mention] when Moses said to his boy [i.e., servant], "I will not cease [traveling] until I reach the junction of the two seas or continue for a long period."

Translation: Saheeh International

Tafsir

And mention when Moses son of Amram ‘Imrān said to his lad Joshua son of Nun Yūsha‘ bin Nūn who used to follow him around serve him and acquire knowledge from him ‘I will not give up I will not stop journeying until I have reached the juncture of the two seas — the point where the Byzantine sea and the Persian sea …

Excerpt from Tafsir al-Jalalayn, commentary on 18:60, via Quran.com. The reader has the full commentary and 9 more tafsirs.