Parashas Shemos
וַיֹּ֤אמֶר אֱלֹהִים֙ אֶל־מֹשֶׁ֔ה אֶֽהְיֶ֖ה אֲשֶׁ֣ר אֶֽהְיֶ֑ה וַיֹּ֗אמֶר כֹּ֤ה תֹאמַר֙ לִבְנֵ֣י יִשְׂרָאֵ֔ל אֶֽהְיֶ֖ה שְׁלָחַ֥נִי אֲלֵיכֶֽם׃
And God said to Moses, “Ehyeh-Asher-Ehyeh,”iEhyeh-Asher-Ehyeh Meaning of Heb. uncertain; variously translated: “I Am That I Am”; “I Am Who I Am”; “I Will Be What I Will Be”; etc. continuing, “Thus shall you say to the Israelites, ‘Ehyeh sent me to you.’”
Torah Temimah points to Berachos 9b (verified).
אהיה אשר אהיה — אמר ליה הקב״ה למשה: לך אמור להם לישראל, אני הייתי עמכם בשעבוד זה ואני אהיה עמכם בשעבוד מלכיות. אמר לפניו: רבש״ע, דיה לצרה בשעתה.
English: “I Will Be What I Will Be” — the Holy One said to Moshe: Go say to Israel, “I was with you in this bondage and I will be with you in the bondage of the [later] kingdoms.” Moshe said before Him: Master of the Universe, sufficient is the trouble in its time.
Custom
Chazal teach that the doubled “Eheyeh” originally signaled both the present and future redemptions, but Moshe requested that future suffering not be disclosed yet.