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I will declare for a decree: God said to me, "you are My son. I, today, gave you birth.

Rashi

I will declare for a decree

Said David: It is a set decree and accepted on me to tell this and to make known.

God said to me

By the hands of Nathan and Gad and Samuel.

You are my son

A head for Israel which are called "My son, My first-born", and they will last by your hand, as is says by Abner: (2 Samuel 3:18) for God spoke etc. 'By the hand of my servant David, I will save my people Israel'...

I, today

...

I gave you birth

...

Ibn Ezra

I will declare

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I, today, gave you birth ...