1 Corinthians 15:4

And that he was buried, and that he rose again the third day according to the scriptures (KJV)

Then: The New Testament was still being written so "according to the scriptures" means the Old Testament. After Paul saw Christ and he got his sight back, he went away for at least three years to study, learn, and understand the Old Testament teachings of Christ (Gal. 1:16-18). He also talked with others so the "creed" words of "buried, "rose again" and "third day" are not original with Paul (Baker, p. 213). He knew the Corinthians would understand them so he didn't need to explain.

Now: Christians still use these scriptural (New Testament) words to focus on the Christ, God's love and his plan for our salvation. Being buried has no meaning, but for the resurrection. The resurrection's credibilty and impact come from Christ's sacrifice in letting himself be crucified, then buried. They merge in the minds of believers to become one continuous event.

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.