Tafsir of Al-Anbiya' 21:56

قال بل ربكم رب السماوات واﻻرض الذي فطرهن

He said: "Nay, your Lord is the Lord of the heavens and the earth, Who created them...'' meaning, your Lord, beside Whom there is no other god, is the One Who created the heavens and the earth and all that they contain; He is the One Who initiated their creation; He is the Creator of all things.

وأنا على ذلكم من الشاهدين

and to that I am one of the witnesses. means, and I bear witness that there is no God other than Him and no Lord except Him