In Isaiah 37 this chapter of the Bible. It tells the story of King Hezekiah, who was distressed after hearing the words of the king of Assyria’s servants, who had blasphemed God. Hezekiah sent his servants to the prophet Isaiah, who told them not to be afraid and that God would send a spirit upon the king of Assyria, causing him to return to his own land and fall by the sword. This is the verse I thought of knowing Isaiah told them to not be afraid.
(Trust in the Lord, and do good; so shalt thou dwell in the land, and verily thou shalt be fed. Psalm 37:3 KJV)

It reads in Isaiah 37:1-7 NKJV> And so it was, when King Hezekiah heard it, that he tore his clothes, covered himself with sackcloth, and went into the house of the Lord. Then he sent Eliakim, who was over the household, Shebna the scribe, and the elders of the priests, covered with sackcloth, to Isaiah the prophet, the son of Amoz. And they said to him, “Thus says Hezekiah: ‘This day is a day of trouble and rebuke and blasphemy; for the children have come to birth, but there is no strength to bring them forth. It may be that the Lord your God will hear the words of the Rabshakeh, whom his master the king of Assyria has sent to reproach the living God, and will rebuke the words which the Lord your God has heard. Therefore lift up your prayer for the remnant that is left.’ ” So the servants of King Hezekiah came to Isaiah. And Isaiah said to them, “Thus you shall say to your master, ‘Thus says the Lord: “Do not be afraid of the words which you have heard, with which the servants of the king of Assyria have blasphemed Me. Surely I will send a spirit upon him, and he shall hear a rumor and return to his own land, and I will cause him to fall by the sword in his own land.”