The Quran

اسلُك يَدَكَ في جَيبِكَ تَخرُج بَيضاءَ مِن غَيرِ سوءٍ وَاضمُم إِلَيكَ جَناحَكَ مِنَ الرَّهبِ ۖ فَذانِكَ بُرهانانِ مِن رَبِّكَ إِلىٰ فِرعَونَ وَمَلَئِهِ ۚ إِنَّهُم كانوا قَومًا فاسِقينَ

Muhammad Asad

"[And now] put thy hand into thy bosom: it will come forth [shining] white, without blemish.¹ And [henceforth] hold thine arm close to thyself, free of all fear.² "These, then, shall be the two signs [of thy bearing a message] from thy Sustainer³ unto Pharaoh and his great ones - for, behold, they are people depraved!"

See note 85 on 7:108.
As pointed out by Zamakhshari, the above idiomatic sentence is a metonym recalling a 'well-known gesture of terror - the involuntary stretching-forth of one's hands or arms when suddenly faced with something terrifying; conversely, the "holding of one's arm [lit., "wing"] close to oneself" is expressive of freedom from fear. In the present instance, the phrase echoes the concluding words of verse 31 - "behold, thou art of those who are secure [in this world and in the next]".
The "two signs" (burhanan) may be understood as Moses' ability to remain, by virtue of his certainty of God's omnipresence, forever free of all physical or moral fear, as well as his ability to show that appearance and reality are not always identical.