تفسير Ma'arif al-Qur'an: سورة طه، الآية 64

فَأَجْمِعُوا كَيْدَكُمْ ثُمَّ ائْتُوا صَفًّا (So, make your plot firm and forth in one row - 20:64). Formation of troops in ranks in battle has an unsettling effect on the enemy, so the Egyptian magicians arranged themselves in ranks for the contest.