1 Chronicles (Author Compiled by Isaiah and Ezra from records of Nathan, Gad, Iddo, Ahijah, Jehu and others.)
29:29And the acts of David the king, first and last, behold, they are written in the book of Samuel the seer, and in the book of Nathan the prophet, and in the book of Gad the seer;
Original from The Bible Foundation - bf.org. They claim public domain status for their original text.