Psalms 118:4

Those who fear Hashem will now say that His kindness lasts forever.

This verse is recited as part of Hallel following verses saying that Israel and “the house of Aaron” (i.e., the kohanim) will acknowledge that G-d’s kindness is eternal. Who, specifically, are these people who fear Hashem? Rashi on this verse says that the ones who fear Hashem are the Levites. In Exodus 32:26, during the incident of the golden calf, Moshe called out, “Whoever is on G-d’s side, come to me!” We are then told that “all the sons of Levi gathered themselves around him.” Furthermore, Rashi says that the Levites fought against those who wanted to mutiny and return to Egypt in Numbers chapter 14. Therefore, we see that the Levites are those who fear G-d. (Accordingly, verses 2-4 would appear to refer to Israelites, kohanim and Leviim, respectively.)