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See note [85] on 7:108 .
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.
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]".
See footnote for 20:22.
When Moses put his arm through the opening in his collar again, his hand returned to its original colour.
Literally, "draw thy wing close to thy side, (away) from fear". When a bird is frightened, it ruffles its wings and prepares to fly away, but when it is calm and composed, it sits with its wings drawn close to its sides, showing a mind secure from danger. Cf. also n. 2550 to xx. 22.