There is a seemingly well known chazal "Ein Melech B'Lo Am" there is no king without a people. Which is supposed to indicate that God cannot truly be king without subjects. Where is this chazal found (if at all)?
I don't know if this is the earliest source, but it is pretty early. In Pirkei deRabbi Eliezer Chapter 3 the Torah asks God the following question:
רבון כל העולם אם אין צבא למלך ואם אין מחנה למלך על מה הוא מולך, אם אין עם לקלס למלך אי זה הוא כבודו של מלך? Master of the Universe, if a king does not have armies or camps then what is he king of? If there is no nation to praise the king, then where is his honor?
Rabbeinu Bechaye says it (using those words) in his commentary on the Torah, Bereshit 38:30. It is a long entry, but the relevant portion is found towards the end (top of the first column here). He also says it in his introduction to Parshat Balak (about 15 lines in here).
He also brings it up in another Sefer of his called Kad HeKemach (Rosh Hashana (2) d"h V'od).