Midrash su Salmi 100:1

Thus, it is written concerning the nations: Why are all the nations in an uproar? And why do the people mutter in vain? (Ps. 2:1). This entire psalm alludes to the nations. For at the conclusion of this psalm, it is written: Serve the Lord with fear, and rejoice with trembling. Do homage in purity, etc. (ibid., vv. 11–12). David said to them: Take heed, lest you deviate from the right path, and He be angry, and ye perish in the way (Ps. 2:12), for He might become angry with you, even over a trivial matter. In reference to Israel, however, it is written: Serve the Lord with gladness; come before His presence with thanksgiving (Ps. 100:2). This psalm was undoubtedly said with reference to Israel, for David composed it in thanksgiving, as it is written: A psalm of thanksgiving (ibid., v. 1).