The New Covenant is a biblical interpretation which was originally derived from a phrase which is contained in the Book of Jeremiah, in the Hebrew Bible.
Moses Speaks to the Children of Israel (illustration from Hartwell James's The Boys of the Bible)
The Book of Jeremiah is the second of the Latter Prophets in the Hebrew Bible, and the second of the Prophets in the Christian Old Testament. The superscription at chapter Jeremiah 1:1–3 identifies the book as "the words of Jeremiah son of Hilkiah". Of all the prophets, Jeremiah comes through most clearly as a person, ruminating to his scribe Baruch about his role as a servant of God with little good news for his audience.
Rembrandt van Rijn, Jeremiah Lamenting the Destruction of Jerusalem (c. 1630)
A high-resolution scan of the Aleppo Codex containing the Book of Jeremiah (the sixth book in Nevi'im)