So what part of the Bible are Gods words?
None of it. (Note: There will be Christians who disagree with me with regard to this statement.) The Bible is not a book dictated by God, but a book written by men who were inspired to write of God as they best understood his revelation. (Well, I shouldn't have said none of it, there a places where the prophets report that they are saying what God told them to say. In those cases the words of the prophets and God's words would have been the same.)
And all of it. All of it has it's source in God's revelation to mankind. But it isn't a book like the Qur'an which claims to be the dictated work of God. If that is what you are looking for, then you're going to be disappointed with the Bible. But it still is a God's instrument for guiding people into relationship with him and able to direct us in the practice of our faith and life.
For instance, when it says, "In the beginning, God created the heavens and the earth..." that isn't God speaking, that is the voice of Moses (or another ancient Jewish author) who expresses his understanding of the origins of the earth as he best understands it based on his knowledge of God as experienced in his own life. Tradition holds that such verses are the product of Moses' hand (or at least his oversight in compilation), then though they may not be the dictated words of God, they still bear the full force of the weight of God who revealed himself to Moses.
Other passages such as the stories of Samuel, Saul, or David in the historical books of Samuel, Kings and Chronicles likewise have similar marks of being the work of inspired men who were nonetheless very human writers. The books of Matthew, Mark, Luke, and John are not meant to even be received as biographies, but have the unique category of "Gospel" a term taken from the opening of Mark's book: "The beginning of the gospel about Jesus Christ, the Son of God" (Mark 1:1) meaning a story of the good news, as it says,
about Jesus Christ, not a record of the life of Jesus or even of his teachings. So, these aren't God's words to us, but man's words about God and what God has done, is doing, and will continue to do, plus some guidance for holy living found in the letters perserved by the church and later adopted as scripture.
Of course, the Hadith of the Prophet (p) is hardly the words of God either (nor does it claim to be), but that doesn't keep Muslims from viewing it as authoritative with regard to their faith and practice. So, why should Muslims be surprised that we can view a book of our scriptures as authoritative for faith and practice even if it isn't the dictated work of God to humankind.