Was God's word written in vain?

Matthew 4:4 'But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.'

1 Samuel 15:23 'For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from being king.'