Psalms 34:4

The Talmud in Brachos (45a) derives two laws from this verse, which is recited by the chazzan upon receiving the Torah. First, a zimmun for bentching (the call for grace after meals) requires a quorum of three because “gadlu” (magnify) is plural (minimum two) and “iti” (with me) is another one. Also, it derives that one who answers “amen” should not do so louder than the one who recited the blessing, as we are meant to exalt G-d together, not in competition.