Midrasz do Diwrej ha-jamim II 20:20

R. Nechemiah says: If one takes upon himself a single mitzvah in (true) faith, he is worthy of having the Holy Spirit repose upon him. And thus do we find with Moses, David, and Devorah that (in the fulness of their faith) they chanted song and the Holy Spirit reposed upon them. And thus do you find that Israel were redeemed from Egypt only in reward for their faith, viz. (Exodus 4:31) "And the people believed, etc." And thus is it written (Psalms 31:24) "The L rd guards the faithful." The faithfulness of the fathers is brought to remembrance, viz. (Exodus 17:12) "And Aaron and Chur supported his (Moses') hands, and his hands were (lifted in) faithfulness until the sun set." And it is written (Psalms 108:20) "This is the gate to the L rd. Tzaddikim (faithful in prayer) will enter through it." What is written of the "believers"? (Isaiah 26:2) "Open the gates and let there enter a nation that is (a) tzaddik, a keeper of faith" — this gate is to be entered by all believers. (Psalms 92:2-5) "It is good to praise the L rd and so sing to Your exalted name. To proclaim in the morning Your lovingkindness, and Your faithfulness in the nights. With a ten-stringed harp, with voice and lyre together. For You have gladdened me by Your deeds, O L rd. At the work of Your hands I will sing." What made him worthy of this joy? The belief held by our fathers in this world, which is all "night." Thus, "to proclaim in the world (to come) Your lovingkindness and Your faithfulness in the 'nights'" (i.e., this world). And thus with Yehoshafat (II Chronicles 20:20) "And they rose early in the morning and they went to the desert of Tekoa. And as they went forth Yehoshafat arose and said: 'Hear me, Judah and dwellers of Jerusalem: Believe in the L rd your G d and you will rest assured. Believe in His prophets and you will succeed.'" And it is written (Jeremiah 5:3) "O L rd, are Your eyes not (looking) for belief?" (Habakkuk 2:4) "And the tzaddik in his belief shall live", and (Eichah 3:23) "They (Your mercies) are renewed every morning — Great is Your faithfulness." And thus do you find that the exiles are gathered in, only in reward for faith, as it is written (Song of Songs 4:8) "With Me, from Levanon, My bride; With Me from Levanon shall you come (out of exile). You shall look (upon the redemption) merosh amanah" (homiletically: "in the merit of emunah [belief]). (Hoshea 2:22) "And I will betroth you to Me forever … and I will betroth you to Me with emunah."