I just took a look at some mishnah manuscripts from Argon's links in this answer. The word appears three times in Kesubos 7:6. The language there is very similar to the language used during kiddushin and may even be the source, although דת משה ויהודית/ישראל in the two cases are not referring to exactly the same laws.
The Kaufmann A50, a Western manuscript ...
Shemot 7:14 says:
וַיֹּ֤אמֶר ה' אֶל־משֶׁ֔ה כָּבֵ֖ד לֵ֣ב פַּרְעֹ֑ה מֵאֵ֖ן לְשַׁלַּ֥ח
The Lord said to Moses, "Pharaoh's heart is heavy; he has refused to
let the people out.
The word “kaveid” means “heavy” although I believe some translate this here as "stubborn" in the Torah. Why not say his heart was “kashah”: “hardened” or “ikeish”: "...