The Stick of Musa turned into a Snake

This was a proof from Allah for Musa and a great miracle. This was something that broke through the boundaries of what is considered normal, thus, it was a brilliant evidence that none but Allah could do. It was also a proof that no one could come with the likes of this (from mankind) except a Prophet who was sent (by Allah). Concerning Allah's statement,

وَمَا تِلْكَ بِيَمِينِكَ يمُوسَى

Commentary

وَمَا تِلْكَ بِيَمِينِكَ يَا مُوسَىٰ The question: "What is in your right hand, 0 Musa"? - 20:17 marks the beginning of the special favour which Allah Ta` ala showed towards Sayyidna Musa (علیہ السلام) in order to dispel the fear created by the unusual sights that he had seen and the Divine Word that he had heard. The style in which Allah Ta` ala addressed him was friendly, inquiring from him what he held in his hand. Besides there was a subtle purpose in the question, namely to make him aware that what he held in his hand was a staff made of wood and nothing more. And when he had satisfied himself again that it was indeed a wooden staff, then the miracle of turning it into a serpent was revealed, thus precluding any doubt in his mind that in the darkness of the night he might have picked up a serpent instead of his staff.