Happy is the believer
Happy is the person who believes. By extension: the expression is used to cast doubt on the truth of things that have been heard, as if to say they are not to be believed.
("And happy is he that trusteth in the LORD" – Proverbs 16:20, and likewise: "Rabbi Nehemiah said: By what merit did they sing? By the merit of faith, for they first believed, as it is said 'And the people believed' (Exodus 4), 'You are beautiful from the beginning of faith' (Song of Songs 4), by the merit of faith they merited to sing, as it is said 'Then Moses sang'" – Midrash Tanchuma (Buber), Parashat Beshalach, section 11, and likewise: "You bring up in my heart the words of the verse: 'But the righteous shall live by his faith.' Happy is the believer" – Y. Burla / Longings, 164)
Translation
Expression (Hebrew)
אַשְׁרֵי הַמַּאֲמִין
Definition (Hebrew)
מְאֻשָּׁר הוּא הָאָדָם הַמַּאֲמִין. בְּהַשְׁאָלָה: הַבִּטּוּי בָּא לְהַטִּיל סָפֵק בִּנְכוֹנוּת דְּבָרִים שֶׁנִּשְׁמְעוּ כְּבָא לְהַגִּיד שֶׁאֵין לְהַאֲמִין לָהֶם.
("וּבוֹטֵחַ בָּהּ' אַשְׁרָיו"- מִשְׁלֵי טז כ, וְכֵן:"אָמַר ר' נְחֶמְיָה: בִּזְכוּת מָה אָמְרוּ שִׁירָה, בִּזְכוּת הָאֱמוּנָה שֶׁהֶאֱמִינוּ תְּחִלָּה, שֶׁנֶּאֱמַר וְיֵאָמֵן הָעָם (שֵׁמוֹת ד), תָּשׁוּרִי מֵרֹאשׁ אֲמָנָה (שִׁיר הַשִּׁירִים ד), בִּזְכוּת הָאֱמוּנָה זָכוּ לוֹמַר שִׁירָה שֶׁנֶּאֱמַר אָז יָשִׁיר מֹשֶׁה"- מִדְרָשׁ תַּנְחוּמָא (בּוּבֶּר) פָּרָשַׁת בְּשַׁלַּח סִימָן יָא, וְכֵן:"מַעֲלֵה אַתָּה עַל לִבִּי דִּבְרֵי הַפָּסוּק: וְצַדִּיק בֶּאֱמוּנָתוֹ יִחְיֶה.אַשְׁרֵי הַמַּאֲמִין"-י.בּוּרְלָא/ כִּסּוּפִים, 164)