Romans 10:9-10 KJV

9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.

Mark 11:22-23 KJV

22 And Jesus answering saith unto them, Have faith in God.
23 For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.

Proverbs 18:20 AMPC

20 A man’s [moral] self shall be filled with the fruit of his mouth; and with the consequence of his words he must be satisfied [whether good or evil].