Rashi on Deuteronomy 9:10/רש"י על דברים ט׳:י׳

1לוחת TABLETS — This word is written without a ו before the ת, so that it may be read לוחת (a singular form), to indicate that both of them were alike (Midrash Tanchuma, Eikev 10; cf. Rashi on Exodus 31:18 and Note thereon).