Midrash su Salmi 118:24

Another matter, “let us exult and rejoice in you.” Rabbi Avin began: “This is the day that the Lord has made, we exult and rejoice in Him [bo]” (Psalms 118:24). Rabbi Avin said: We do not know regarding what to rejoice, whether the day, or the Holy One blessed be He.¹⁸¹The word bo in the phrase “we exult and rejoice bo” can alternatively be translated “in it,” meaning the day, or “in Him,” referring to God. Solomon came and specified: “Let us exult and rejoice in You,” in the Holy One blessed be He. In You, in Your salvation; in You, in Your Torah; in You, in fear of You.¹⁸²This is in line with the verse: “Serve the Lord with fear and rejoice with trembling” (Psalms 2:11) (Rabbi David Luria). Rabbi Yitzḥak said: “In You [bakh],” with the twenty-two letters that You wrote for us in the Torah, beit, two, kaf, twenty, that is bakh.