Second Book of Paralipomenon (Author Compiled by Isaias and Esdras from records of Nathan, Gad, Addo, Ahias, Jehu and others.)
20:21deditque consilium populo et statuit cantores Domini ut laudarent eum in turmis suis et antecederent exercitum ac voce consona dicerent confitemini Domino quoniam in aeternum misericordia eius
Original from The Bible Foundation - bf.org. They claim public domain status for their original text.