Shir HaShirim Rabbah 4:3

כְּחוּט הַשָּׁנִי שִׂפְתוֹתַיִךְ, בְּשָׁעָה שֶׁאָמְרוּ אֶת הַשִּׁירָה (שמות טו, א): אָז יָשִׁיר משֶׁה. וּמִדְבָּרֵךְ נָאוֶה, שֶׁמַּרְאִין בָּאֶצְבַּע וְאוֹמְרִים (שמות טו, ב): זֶה אֵלִי וְאַנְוֵהוּ, בְּאוֹתָהּ שָׁעָה הִתְחִיל משֶׁה מְשַׁבְּחָן: כְּפֶלַח הָרִמּוֹן רַקָּתֵךְ, הָרֵיקָן שֶׁבָּכֶם רָצוּף מִצְווֹת כָּרִמּוֹן הַזֶּה, אֵין צָרִיךְ לוֹמַר מִבַּעַד לְצַמָּתֵךְ עַל הַצְּנוּעִין וְהַמְצֻמָּתִין שֶׁבָּכֶם. “Your lips are like a scarlet thread, and your speech is lovely; your temple is like a pomegranate slice behind your braid” (Song of Songs 4:3).
“Your lips are like a scarlet thread” – when they sang the song: “Then sang Moses [and the children of Israel]” (Exodus 15:1). “Your speech is lovely [naveh]” – that they point with their finger and say: “This is my God and I will glorify Him [ve’anvehu]” (Exodus 15:2). At that moment, Moses began praising them: “Your temple [rakatekh] is like a pomegranate slice” – even the empty [hareikan] among you is packed with mitzvot like this pomegranate. It goes without saying: “Behind your braid [letzamatekh]” – regarding the modest and the fervent [metzumatin] among you.