In the Vatican Library at Rome.
It contains the whole Bible, written in the fourth century, and is (apart from the papyri) the oldest and generally the best extant copy of the Septuagint.
In Codex Vaticanus the Book of the Prophet Malachi preceeds that of Isaiah. This is folio 1002r. It contains the last passages of Malachi beginning part way through Mal.3:17.
Malachi is the last book of the canonical Old Testament. It ends with a promise: "And, behold, I will send to you Elias the Thesbite, before the great and glorious day of the Lord comes; who shall turn again the heart of the father to the son, and the heart of a man to his neighbour." (Elias the Thesbite is the prophet Elijah!)
The book of the prophet Isaiah starts at the top of column 3 running to verse 7. The passages in their context are HERE and HERE.
Description from 'Our Bible & the
Ancient Manuscripts' by Sir Frederick Kenyon (1895 - 4th Ed. 1939)
Ch.5.
top