Rava expounded: as a reward for Abraham saying "from a thread to a shoelace" (Genesis 14:23), his children merited two commandments — the thread of blue [tekhelet] and the strap of the phylacteries [tefillin]. Now, the strap of the tefillin is well understood, as it is written, "and all the peoples of the earth shall see that the name of the LORD is called upon you, and they shall fear you" (Deuteronomy 28:10), and it is taught that Rabbi Eliezer the Great says: these are the tefillin of the head. But the thread of blue — what is its significance? As it is taught: Rabbi Meir used to say, why is blue [tekhelet] singled out from all the kinds of dyes? Because blue resembles the sea, and the sea resembles the sky [the firmament], and the sky resembles the Throne of Glory, as it is said, "and they saw the God of Israel, and under His feet there was as it were a paved work of sapphire" (Exodus 24:10), and it is written, "as the appearance of a sapphire stone, the likeness of a throne" (Ezekiel 1:26).
Rava on the Thread of Blue, the Tefillin Strap, and the Throne of Glory
Yalkut Shimoni on Torah 76:3
דָּרַשׁ רָבָא, בִּשְׂכַר שֶׁאָמַר אַבְרָהָם אִם מִחוּט וְעַד שְׂרוֹךְ נַעַל זָכוּ בָּנָיו לִשְׁתֵּי מִצְווֹת, חוּט שֶׁל תְּכֵלֶת וּרְצוּעָה שֶׁל תְּפִלִּין בִּשְׁלָמָא רְצוּעָה שֶׁל תְּפִלִּין דִּכְתִיב, "וְרָאוּ כָּל עַמֵּי הָאָרֶץ כִּי שֵׁם ה' נִקְרָא עָלֶיךָ וְיָרְאוּ מִמְּךָּ", וְתַנְיָא, רַבִּי אֱלִיעֶזֶר הַגָּדוֹל אוֹמֵר, אֵלוּ תְּפִלִּין שֶׁבָּרֹאשׁ. אֶלָּא חוּט שֶׁל תְּכֵלֶת מַאי הִיא דְּתַנְיָא, הָיָה רַבִּי מֵאִיר אוֹמֵר, מַה נִשְׁתַּנָּה תְּכֵלֶת מִכָּל מִינֵי צִבְעוֹנִין מִפְּנֵי שֶׁהַתְּכֵלֶת דּוֹמֶה לְיָם וְיָם דּוֹמֶה לְרָקִיעַ וְרָקִיעַ דּוֹמֶה לְכִסֵּא הַכָּבוֹד שֶׁנֶּאֱמַר, (שמות כד, י) "וַיִּרְאוּ אֵת אֱלֹהֵי יִשְׂרָאֵל וְתַחַת רַגְלָיו כְּמַעֲשֶׂה לִבְנַת הַסַּפִּיר", וּכְתִיב, (יחזקאל א, כו) "כְּמַרְאֶה אֶבֶן סַפִּיר דְּמוּת כִּסֵּא".