Sources for Adon Olam's “He was, He is, and He shall be”
רְא֣וּ ׀ עַתָּ֗ה כִּ֣י אֲנִ֤י אֲנִי֙ ה֔וּא וְאֵ֥ין אֱלֹהִ֖ים עִמָּדִ֑י אֲנִ֧י אָמִ֣ית וַאֲחַיֶּ֗ה מָחַ֙צְתִּי֙ וַאֲנִ֣י אֶרְפָּ֔א וְאֵ֥ין מִיָּדִ֖י מַצִּֽיל׃
See, then, that I, I am the One;There is no god beside Me.I deal death and give life;I wounded and I will heal:None can deliver from My hand.
כַּד יִתְגְלֵי מֵימְרָא דַיְיָ לְמִפְרוֹק יַת עַמֵיהּ יֵימַר לְכָל עַמְמַיָא חֲמוֹן כְּדוֹן אֲרוּם אֲנָא הוּא דַהֲוֵויתִי וַהֲוִית וַאֲנָא הוּא דְעָתִיד לְמֶהֱוֵי וְלֵית אֱלָהָא חוֹרַן בַּר מִינִי אֲנָא בְּמֵימְרִי מֵמִית וּמָחֵי מָחִיתִי יַת עַמָא בֵּית יִשְרָאֵל וַאֲנָא אָסֵי יַתְהוֹן בְּסוֹף יוֹמַיָא וְלֵית דִמְשֵׁזִיב מִן יְדַי גוֹג וּמַשִׁרְיָיתֵיהּ לִמְסַדְרָא סִדְרֵי קְרָבָא עִמְהוֹן
When the Word of the Lord shall reveal Himself to redeem His people, He will say to all the nations: Behold now, that I am He who Am, and Was, and Will Be, and there is no other God beside Me: I, in My Word, kill and make alive; I smite the people of the Beth Israel, and I will heal them at the end of the days; and there will be none who can deliver them from My hand, Gog and his armies whom I have permitted to make war against them.

And this same title for God also shows up in liturgy in the song “Adon Olam”:
אֲדוֹן עוֹלָם אֲשֶׁר מָלַךְ, בְּטֶֽרֶם כָּל יְצִיר נִבְרָא: לְעֵת נַעֲשָׂה בְחֶפְצוֹ כֹּל, אֲזַי מֶֽלֶךְ שְׁמוֹ נִקְרָא: וְאַחֲרֵי כִּכְ֒לוֹת הַכֹּל, לְבַדּוֹ יִמְלֹךְ נוֹרָא: וְהוּא הָיָה וְהוּא הֹוֶה, וְהוּא יִהְיֶה בְּתִפְאָרָה: וְהוּא אֶחָד וְאֵין שֵׁנִי, לְהַמְשִׁיל לוֹ לְהַחְבִּֽירָה: בְּלִי רֵאשִׁית בְּלִי תַכְלִית, וְלוֹ הָעֹז וְהַמִּשְׂרָה: וְהוּא אֵלִי וְחַי גּוֹאֲלִי, וְצוּר חֶבְלִי בְּעֵת צָרָה: וְהוּא נִסִּי וּמָנוֹס לִי, מְנָת כּוֹסִי בְּיוֹם אֶקְרָא: בְּיָדוֹ אַפְקִיד רוּחִי, בְּעֵת אִישַׁן וְאָעִֽירָה: וְעִם רוּחִי גְּוִיָּתִי, יְהֹוָה לִי וְלֹא אִירָא:
Master of the Universe Who reigned before any creature was created. At the time when all was made by His will, then was His Name proclaimed King. And after all things shall cease to be the Awesome One will reign alone. He was, He is, and He shall be in glory. He is One, and there is no second to compare to Him, to associate [with Him]. Without beginning, without end, power and dominion are His. He is my God and my ever-living Redeemer, the Rock of my destiny on the day of distress. He is my flag and my refuge; He is the portion of my cup on the day I call. Into His hand I entrust my spirit [both] when I sleep and when I awaken. And with my spirit my body [too], Adonoy is with me, I shall not fear.
Adon Olam (Koren Sacks Siddur)Lord of the Universe, who reigned before the birth of any thing;when by His will all things were made, then was His name proclaimed King.And when all things shall cease to be, He alone will reign in awe.He was, He is, and He shall be glorious for evermore.He is One, there is none else, alone, unique, beyond compare;Without beginning, without end, His might, His rule are everywhere.He is my God; my Redeemer lives. He is the Rock on whom I rely —my banner and my safe retreat, my cup, my portion when I cry.Into His hand my soul I place, when I awake and when I sleep.The LORD is with me, I shall not fear; body and soul from harm will He keep.