This phrase appears at least nine times in the Bible (KJV). Moses often says it to Joshua or Israel, Joshua sas it to Israel, David to Solomon, and God to others. I really like to remember it, and it usually goes with the idea that God is with us. If God is with us, our own strength is irrelevant.
Deu 31:6 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.
Deu 31:7 And Moses called unto Joshua, and said unto him in the sight of all Israel, Be strong and of a good courage: for thou must go with this people unto the land which the LORD hath sworn unto their fathers to give them; and thou shalt cause them to inherit it.
Deu 31:23 And he gave Joshua the son of Nun a charge, and said, Be strong and of a good courage: for thou shalt bring the children of Israel into the land which I sware unto them: and I will be with thee.
Jos 1:6 Be strong and of a good courage: for unto this people shalt thou divide for an inheritance the land, which I sware unto their fathers to give them.
Jos 1:9 Have not I commanded thee? Be strong and of a good courage; be not afraid, neither be thou dismayed: for the LORD thy God is with thee whithersoever thou goest.
Jos 1:18 Whosoever he be that doth rebel against thy commandment, and will not hearken unto thy words in all that thou commandest him, he shall be put to death: only be strong and of a good courage.
Jos 10:25 And Joshua said unto them, Fear not, nor be dismayed, be strong and of good courage: for thus shall the LORD do to all your enemies against whom ye fight.
1Ch 22:13 Then shalt thou prosper, if thou takest heed to fulfil the statutes and judgments which the LORD charged Moses with concerning Israel: be strong, and of good courage; dread not, nor be dismayed.
1Ch 28:20 And David said to Solomon his son, Be strong and of good courage, and do it: fear not, nor be dismayed: for the LORD God, even my God, will be with thee; he will not fail thee, nor forsake thee, until thou hast finished all the work for the service of the house of the LORD.