The reference is to that even of Moses when sorcerers were sent for to confront him and they showed their sorcery by throwing ropes and sticks on the ground and Moses felt afraid. Thus, the Qur'an records:
. . . it seemed to him (Moses), by their sorcery as if they were running.Then Moses felt in himself a fear. We said: Fear not! Verily thou art the uppermost. (20:66-68)
Amir al-mu'minin says that the ground for Moses fear was not that since he saw ropes and sticks moving he might have entertained fear for his life but the cause of his fear was lest people be impressed with this sorcery and get astray, and untruth might prevail on account of this craft. That is why Moses was not consoled by saying that his life was safe but by saying that he would prove superior, and his claim would be upheld. Since his fear was for the defeat of the truth and victory of the untruth, not for his own life, the consideration was given to him for the victory of truth and not for the protection of his life. Amir al-mu'minin also means that he too had the same fear viz. that the people should not be caught in the trap of these (Talhah, az-Zubayr, etc.) and fail into misguidance by getting astray from the true faith. Otherwise, he himself never feared for his own life.