Tafsir of Ash-Shu`ara' 26:19

"(And you did your deed which you did)"—meaning the killing of the Copt. He rebuked him with it after having emphasized and magnified it upon him through the ambiguity contained in the relative pronoun, intending thereby to disparage his prophethood, peace be upon him. Al-Sha'bi read it as fi’lataka with a kasra on the fa’, intending the manner [of the action], as it was a killing by a thrust, while the majority read it with a fatha to intend the single instance.

"(And you were of the ungrateful ones)"—meaning: of those ungrateful for My blessing, since you intentionally killed a man of my elite [subjects], as is narrated from Ibn Zayd. Or: "And you were, at that time, among the group of people whom you now claim to be disbelievers," as is narrated from Al-Suddi. This judgment from him was based on what he knew of his outward state, peace be upon him, at that time due to his mingling with them and his taqiyya (precautionary dissimulation) among them by not condemning them. Otherwise, the prophets, peace be upon them, are infallible from disbelief both before and after their prophethood. It is also said that this was a fabrication on his part against him, peace be upon him; however, this is considered unlikely, for if he had known of his belief at the time, he would have imprisoned or killed him. Under both interpretations, the sentence occupies the position of a state (hal) from one of the two ta's in the two preceding verbs.

It is also permitted that this be an inaugural judgment upon him, peace be upon him, that he was among the disbelievers in his divinity, as is narrated from Al-Hasan; or among those who disbelieve in their religion, given that they had gods they worshipped; or among those ungrateful for blessings who are accustomed to belittling them, and whoever is accustomed to that, such a crime would not be an innovation for him. In this case, the sentence is initial (musta'nafa) or coordinated with what precedes it.

In my view, the former—making the sentence a state—is preferable, so that it may be in the same style as its counterpart in the response, given that the state is definitive there. Since the speech of the accursed one contained two matters, he (Moses), peace be upon him, set out to refute them in a manner of inverted distribution (laff wa nashr): he first refuted that with which he had rebuked him in disparagement of his prophethood—namely, his saying "And you did your deed which you did"—out of concern and attention for it, and that is through what the Exalted and Almighty narrated saying: