According to Graham Priest, although it was Kant, in his Critique of Pure Reason, who first noted the natural occurrence of contradiction when we attempt to apply our notions outside of their natural range of application, it was in fact Hegel, in his Logic, who first accepted the legitimacy of contradictory concepts - i.e., that there are true contradictions. In fact, again according to Priest, Hegel held that all of our concepts are contradictory.
Kant considered the application of concepts beyond the bounds of our experience to be illegitimate. While Hegel agreed with Kant that such arguments that end in contradiction proceed by perfectly legitimate reasoning, he found no reason for declaring the applications of concepts in such arguments to be illegitimate. Hegel argued that the distinction between objects of our experience and objects of our thought had no particular ontological significance.
Thus, according to Hegel, if correct reasoning using legitimate applications of certain concepts leads to contradiction, then the concepts are contradictory. And, according to Hegel, since a sound argument must have a true conclusion, there must be contradictions which are true.
Quoting Priest, from his book In Contradiction :
The point I wish to isolate and highlight is Hegel’s contention that our concepts are contradictory, and there are true contradictions.
Conifold's excellent answer is probably the answer you are looking for. However, I note that Priest, who defines dialetheia as true contradictions, identifies Hegel's Logic as the first to accept their legitimacy.