Someone in a Jewish bookstore told me it was but wasn't able to locate the verse. I've always found it odd that the name is written out in texts (with vowels even), but we don't pronounce it. Also odd that my name, Yonatan, includes the name and that's obviously meant for articulation. So, mitzvah or cultural norm or particular interpretation of a commandment?
The biblical source is:
זֶה־שְּׁמִ֣י לְעֹלָ֔ם וְזֶ֥ה זִכְרִ֖י לְדֹ֥ר דֹּֽר--This shall be My name forever, My appellation for all eternity. [Ex. 3:15]
לְעֹלָ֔ם (le-olam--forever) is written without a vav, which means it can be read le-allem -- to hide. This is what Rashi says, quoting Talmud and Midrash:
זה שמי לעלם THIS IS MY NAME FOR EVER — The last word is written without ו (so that it may be read לְעַלֵּם and it would mean “this is My Name which is to be concealed”) to suggest: Conceal it (this Divine Name), so that it shall not be read exactly as it is written (but should be read as אדני; cf. Pesachim 50a; Exodus Rabbah 3:7).
The non-biblical ban on pronouncing HaShem derived from Sanhedrin 10.1 : "Abba Shaul says: Also included in the exceptions is one who pronounces the ineffable name of God as it is written, with its letters." ( אַבָּא שָׁאוּל אוֹמֵר, אַף הַהוֹגֶה אֶת הַשֵּׁם בְּאוֹתִיּוֹתָיו )
In The Tanakh, swearing "חַי־יְהֹוָ֑ה" established truth statements - as instructed by Moshe in Devarim 10 verse 20 : "You shall fear YHVH, your God, worship Him, and cleave to Him and swear by His-Name (אֶת־יְהֹוָ֧ה אֱלֹהֶ֛יךָ תִּירָ֖א אֹת֣וֹ תַֽעֲבֹ֑ד וּב֣וֹ תִדְבָּ֔ק וּבִשְׁמ֖וֹ תִּשָּׁבֵֽעַ)"