The Hand of Musa turning White without any Disease
This is the second sign of Musa. That is Allah has commanded him to place his hand into the opening of his garment, as is clearly stated in another Ayah. It mentioned here merely as a passing reference, saying:
وَاضْمُمْ يَدَكَ إِلَى جَنَاحِكَ
(And press your hand to your side:) Allah said in another Ayah,
(And draw your hand close to your side to be free from fear. These are two signs from your Lord to Fir`awn and his chiefs.) 28:32 Mujahid said,
وَاضْمُمْ يَدَكَ إِلَى جَنَاحِكَ
(And press your hand to your side:) "This means put your palm under your upper arm." When Musa put his hand into the opening of his garment and brought it out, it came out shining as if it were a half moon. Concerning His statement,
وَاضْمُمْ يَدَكَ إِلَىٰ جَنَاحِكَ (And put your hand under your arm - 20:22). The word جَنَاحِ means fore-limb of an animal or wing of a bird. Here Sayyidna Musa (علیہ السلام) was commanded to press his hand under his armpit so that when he brings it out it will shine as brilliantly as the sun, and this will be the second miracle granted to him. This is the meaning given to the words by Sayyidna Ibn ` Abbas ؓ (Mazhari).
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 22–23
The question ‘What do you have in your right hand?’ was meant to awaken Moses’s awareness. Its purpose was to impress afresh on his mind that his stick was but a stick, so that the next moment when, due to a miracle of God, it changed into a snake, he should be fully aware of its significance. Moses’ stick being changed into a snake was a miracle of transformation. But it must be borne in mind that all the things we see on earth are actually in a process of transformation. An acorn planted in soil and watered will one day grow into a mighty oak tree. Vapour condenses and turns into water, and water, when boiled, will turn into steam. Some of these processes are rapid and others are slow. Some are so slow—like the growth of a tree—that they are imperceptible to the human eye. The slower the process, the less strange it seems, because its slowness gives us time to familiarise ourselves with the phenomenon. When Moses’ stick took the shape of a snake, it happened in an instant, and so seemed very strange and startling. The fact is that whatever there is in this world, or whatever the events, all are ‘miracles’ of God, be it the emerging of a sapling from the earth or a stick becoming a snake. ‘Extraordinary’ miracles are shown through prophets to make man take notice of the everyday miracles wrought by God.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.