Exodus 15:18

ה’ יִמְלֹךְ לְעֹלָם וָעֶד

Hashem will reign forever and ever.

This verse, recited before Shemoneh Esrei, comes from the song that the Jews sang after their redemption at the Red Sea. The Ramban explains that this verse is actually a prayer. The Jews at the sea were saying that, just as Hashem demonstrated His dominion over the world by saving them and punishing their oppressors, so may He reveal His kingship in all subsequent generations.