Deuteronomy 8:3
…that he might make you know that man does not live by bread alone, but that man lives by everything that proceeds out of the mouth of the LORD. (Moses addressing the people before they enter the promised land)

Matthew 4:4
But he answered, “It is written, `Man shall not live by bread alone,
but by every word that proceeds from the mouth of God.'”  (Jesus replying to the evil one about turning stones into bread)

These two passages of sacred Scripture took place about 1500 years apart.  They are a great example of what is called in Bible Study the “content and unity of Scripture”. From the Catechism of the Catholic Church #112 : Be especially attentive “to the content and unity of the whole Scripture.” Different as the books which comprise it may be, Scripture is a unity by reason of the unity of God’s plan, of which Christ Jesus is the center and heart, open since his Passover.

It is Jesus himself that gives us this principle for the understanding of scripture. There are many places in the Gospels where Jesus teaches by quoting from the Old Testament.

So where can we find “every word that proceeds from the mouth of God” ? In the Bible, the entire Bible.