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,

﴿وَاضْمُمْ إِلَيْكَ جَنَاحَكَ مِنَ الرَّهْبِ فَذَانِكَ بُرْهَانَـنِ مِن رَّبِّكَ إِلَى فِرْعَوْنَ وَمَلَئِهِ﴾

(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,

﴿تَخْرُجْ بَيْضَآءَ مِنْ غَيْرِ سُوءٍ﴾

(it will come forth white, without any disease) This means without any leprosy, ailment, or disfigurement. This was stated by Ibn `Abbas, Mujahid, `Ikrimah, Qatadah, Ad-Dahhak, As-Suddi and others. Al-Hasan Al-Basri said, "He brought it out, and by Allah, it was as if it were a lamp. From this Musa knew that he had surely met his Lord, the Mighty and Sublime.'' This is why Allah says,

﴿لِنُرِيَكَ مِنْ ءَايَـتِنَا الْكُبْرَى ﴾

(That We may show you (some) of Our greater signs.)