Deuteronomy 31:6 says: Be strong and of a good courage, fear not, nor be afraid of them: for the LORD thy God, he it is that doth go with thee; he will not fail thee, nor forsake thee.
(KJV).
The verse is spoken by Moses to Israel in Deuteronomy 31, before Joshua takes over as leader. Read it in context at Deuteronomy 31:6, or the full chapter at Deuteronomy 31.