﴿ رَبِّ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضِ وَمَا بَيْنَهُمَآ ۖ إِن كُنتُم مُّوقِنِينَ ﴾

“the Sustainer of the heavens and the earth and all that is between them - if you could but grasp it with inner certainty!”

Lord of the heavens and the earth and all that is between them read rabbu’l-samāwāti in the nominative to understand it as a third predicate; or read rabbi’l-samāwāti as a substitution for rabbika ‘your Lord’ if you should be certain O people of Mecca that God exalted be He is the Lord of the heavens and the earth then also be certain that Muhammad (s) is His Messenger.