The Quran

وَدَخَلَ المَدينَةَ عَلىٰ حينِ غَفلَةٍ مِن أَهلِها فَوَجَدَ فيها رَجُلَينِ يَقتَتِلانِ هٰذا مِن شيعَتِهِ وَهٰذا مِن عَدُوِّهِ ۖ فَاستَغاثَهُ الَّذي مِن شيعَتِهِ عَلَى الَّذي مِن عَدُوِّهِ فَوَكَزَهُ موسىٰ فَقَضىٰ عَلَيهِ ۖ قالَ هٰذا مِن عَمَلِ الشَّيطانِ ۖ إِنَّهُ عَدُوٌّ مُضِلٌّ مُبينٌ

Muhammad Asad

And [one day] he entered the city at a time when [most of] its people were [resting in their houses,] unaware of what was going on [in the streets];¹ and there he encountered two men fighting with one another - one of his own people,² and the other of his enemies. And the one who belonged to his own people cried out to him for help against him who was of his enemies - whereupon Moses struck him down with his fist, and [thus] brought about his end. [But then] he said [to himself]: "This is of Satan's doing! Verily, he is an open foe, leading [man] astray!"³

Lit., "at a time of its people's unawareness".
I.e., of the Hebrews.
Regarding the reference to "Satan's doing", see first half of note 16 on 15:17. In the present instance, verses 16-17 seem to indicate that it was the Israelite, and not the Egyptian, who had been in the wrong (cf. next note). Apparently, Moses had come to the assistance of the Israelite out of an instinctive sense of racial kinship without regard to the rights and wrongs of the case; but immediately afterwards he realized that he had committed a grave sin not only by killing, however inadvertently, an innocent person, but also by basing his action on a mere tribal - or, as we would describe it today, racial or national - prejudice. Evidently, this is the purport of the above Qur'anic segment of the story of Moses. Its moral has been stressed and explained by the Prophet on many occasions: cf. his famous saying, "He is not of us who proclaims the cause of tribal partisanship ('asabiyyah); and he is not of us who fights in the cause of tribal partisanship; and he is not of us who dies in the cause of tribal partisanship" (Abu Da'ud, on the authority of Jubayr ibn Mut'im). When he was asked to explain the meaning of "tribal partisanship", the Prophet answered, "It means helping thine own people in an unjust cause" (ibid., on the authority of Wathilah ibn al-Asqa').