Tafsir of Ta-Ha 20:20

(So he threw it) as soon as he was told to throw it, (and behold, it was a snake moving) meaning walking and moving quickly. "Snake" (hayyah) is a generic noun applicable to the small and the large, the female and the male. It had turned, at the moment he threw it, peace be upon him, into a thu'ban (a large serpent), which is the massive type of snake, as clarified by the words of the Exalted: "And behold, it was a serpent manifest." Its comparison to the jann—the thin variety of snake—in His saying, "And when he saw it moving as if it were a jann," refers to its agility and speed of movement, not to the smallness of its body; thus, there is no contradiction.

It is said: It turned, when he threw it, peace be upon him, into a yellow snake the thickness of a staff, then it swelled and grew thick; therefore, it was likened to the jann at one time and called a thu'ban at another, and it was expressed by the general noun for both states. The first [interpretation] is more befitting the context, given the evident demand of the verse we mentioned for it and its distance from [forced] interpretation.

Ahmad and others narrated from Wahb that he, peace be upon him, caught a glimpse after he had thrown it, and behold, it was the greatest serpent eyes had ever seen. It was slithering, seeking, as if it were looking for something it intended to seize. It would pass by a boulder the size of a large camel and swallow it; it would strike the root of a massive tree with one of its fangs and uproot it. Its eyes were like kindled fire. The crook [of the staff] had become a mane with hair like shooting stars, and its two prongs had become like a wide pit, containing molars and fangs that made a grinding sound.

In some reports, it is stated that there were forty cubits between its jaws. When Moses, peace be upon him, witnessed this, he turned and fled, not looking back, and he went until he had gone far, thinking he had outdistanced the snake.