Midrasz do Izajasza 49:1

(Fol. 15) R. Elazar said again: "Not to everyone is it permitted to praise God by rising or by bowing. Kings may do so by rising, and princes by bowing. Kings may do so by rising, as it is said (Is. 49, 1 ) Thus hath said the Lord, the Redeemer of Israel, his Holy One, to Him who is despised of men, to him who is abhorred of nations, to a servant of rulers, kings shall see and arise. Princes may do so by prostrating themselves, as it is said (Ib.) Princes, and they shall prostrate themselves." R. Zera, and according to others Samuel b. Nachmeni, contradicted this: "If the verse read, and princes shall prostrate themselves, it would be correct; but since it reads princes, and they shall prostrate, etc., it implies that they may do both."