"So it shall be, when you are on the verge of battle, that the priest shall approach and speak to the people. And he shall say to them, 'Hear, O Israel: Today you are on the verge of battle with your enemies. Do not let your heart faint, do not be afraid, and do not tremble or be terrified because of them; for the Lord your God is He who goes with you, to fight for you against your enemies, to save you.'"
I can't find any direct quotations of this passage later in the Scriptures, though there may be some. I did find the following speeches and prophecies in the histories:
Abijah: "Now look, God Himself is with us as our head, and His priests with sounding trumpets to sound the alarm against you." 2 Chronicles 13:12
Asa: "Lord, it is nothing for You to help, whether with many or with those who have no power; help us, O Lord our God, for we rest on You, and in Your name we go against this multitude. O Lord, You are our God; do not let man prevail against You!" 2 Chronicles 14:11
Jahaziel: "Listen, all you of Judah and you inhabitants of Jerusalem, and you, King Jehoshaphat! Thus says the Lord to you: 'Do not be afraid nor dismayed because of this great multitude, for the battle is not yours, but God's...' Do not fear or be dismayed; tomorrow go out against them, for the Lord is with you." 2 Chronicles 20:15-17