Charles Ellicott Commentary Psalms 118:1-4

"Oh give thanks unto Jehovah; for he is good; For his lovingkindness [endureth] for ever. Let Israel now say, That his lovingkindness [endureth] for ever. Let the house of Aaron now say, That his lovingkindness [endureth] for ever. Let them now that fear Jehovah say, That his lovingkindness [endureth] for ever." — Psalms 118:1-4 (ASV)

Compare Psalms 115:9-13, where a similar choral arrangement is found.