Parashas Shemos
וַיֹּ֩אמֶר֩ ע֨וֹד אֱלֹהִ֜ים אֶל־מֹשֶׁ֗ה כֹּֽה־תֹאמַר֮ אֶל־בְּנֵ֣י יִשְׂרָאֵל֒ יְהֹוָ֞ה אֱלֹהֵ֣י אֲבֹתֵיכֶ֗ם אֱלֹהֵ֨י אַבְרָהָ֜ם אֱלֹהֵ֥י יִצְחָ֛ק וֵאלֹהֵ֥י יַעֲקֹ֖ב שְׁלָחַ֣נִי אֲלֵיכֶ֑ם זֶה־שְּׁמִ֣י לְעֹלָ֔ם וְזֶ֥ה זִכְרִ֖י לְדֹ֥ר דֹּֽר׃
And God said further to Moses, “Thus shall you speak to the Israelites: GOD [יהוה],jיהוה This divine name (y-h-w-h; traditionally pronounced as Adonai, lit. “[the] Sovereign”) is here associated with the verb hayah “to be.” the God of your fathers—the God of Abraham, the God of Isaac, and the God of Jacob—has sent me to you:This shall be My name forever,This My appellation for all eternity.
Torah Temimah points to R’ Yochanan — Kiddushin 71a (verified).
זה שמי לעלם — א״ר יוחנן: שם בן ארבע אותיות חכמים מוסרין אותו לתלמידיהם פעם אחת בשבוע ואמרי לה פעמים בשבוע.
English: “This is My name forever (le’olam)” — R’ Yochanan said: The four-letter Name the Sages transmit to their disciples once a week, and some say twice a week.
Custom
Chazal read the spelling as an instruction to keep the proper pronunciation of the four-letter Name restricted.