The Role of the Book in the Faith of Paul

The author of Hebrews, I think it was Paul puts these words in the lips of our Lord during his state of faith. The new American standard set 1977 translates this verse as "It is written of me to do thy will or God" It's obvious if you read wider passage that the author puts words in the mouth of our Lord at his first coming,. But he's saying, look, here's what he said.