The Quran

وَقالَ رَجُلٌ مُؤمِنٌ مِن آلِ فِرعَونَ يَكتُمُ إيمانَهُ أَتَقتُلونَ رَجُلًا أَن يَقولَ رَبِّيَ اللَّهُ وَقَد جاءَكُم بِالبَيِّناتِ مِن رَبِّكُم ۖ وَإِن يَكُ كاذِبًا فَعَلَيهِ كَذِبُهُ ۖ وَإِن يَكُ صادِقًا يُصِبكُم بَعضُ الَّذي يَعِدُكُم ۖ إِنَّ اللَّهَ لا يَهدي مَن هُوَ مُسرِفٌ كَذّابٌ

Muhammad Asad

At that, a believing man of Pharaoh's family, who [until then] had concealed his faith,¹ exclaimed: "Would you slay a man because he says, 'God is my Sustainer' - seeing, withal, that he has brought you all evidence of this truth from your Sustainer? Now if he be a liar, his lie will fall back on him; but if he is a man of truth, something [of the punishment] whereof he warns you is bound to befall you: for, verily, God would not grace with His guidance one who has wasted his own self by lying [about Him].²

Cf. the parable of the believer in 36:20-27 and, in particular, the corresponding note 15.
Lit., "a liar". As regards my rendering of musrif as "one who wastes [or "has wasted"] his own self", see note 21 on the last sentence of 10:12. Thus, the anonymous believer spoken of here argues that the message brought by Moses is so convincing that, by itself, it is a proof of his not being "one who wastes his own self" - i.e., destroys himself spiritually - by a spurious claim to divine inspiration.