In Shir HaShirim (5:11), Rashi writes: "רְבָבוֹת הַרְבֵּה קְרוּיִין רְבָבָה, שֶׁנֶּאֱמַר "רְבָבָה כְּצֶמַח הַשָׂדֶה נְתַתִּיךְ": Many myriads are called ‘רְבָבָה’, as it is stated, “Myriads רְבָבָה like the vegetation of the field have I made you.” So the plural word רְבָבוֹת becomes singular (רְ...