The LORD, the God of Israel has spoken.  

Whether a prophecy of destruction as we see in Chapter 21, or a promise such as the one in Isaiah 41:10 So do not fear for I am with you, do not be dismayed, for I am your God.  I will strengthen you and help you, I will uphold you with my righteous right hand, or the promise of salvation and eternal life, when the LORD, the God of Israel has spoken, it will be accomplished.  Ps. 95: 7 for he is our God and we are the people of his pasture, the flock under his care.