Rashi on Exodus 7:14/רש"י על שמות ז׳:י״ד

1כבד — This word should be translated in the Targum by יקיר (his heart is hard — an adjective) and not by אתיקר (his heart has become Hard — a verb, as is given in some editions of Onkelos) because it is an adjective, just as (Exodus 18:18) “the thing is too heavy (כָּבֵד) for thee”.