G-d said to Moses, “Reach out and grasp its tail.” When he reached out and took hold of it, it turned into a staff in his hand.
Exodus 4:4
When Moses told G-d that he doubted that the Jewish people would believe him, G-d told him to cast his staff to the ground. When Moses did so, the staff turned into a snake. By making the staff turn specifically into a snake, G-d hinted to Moses that he was guilty of slander, just as the primordial snake had slandered G-d to Eve.
(The snake convinced Eve that rather than for their own good, G-d forbade Adam and her to eat the fruit of the Tree of Knowledge because He was jealously reserving for Himself the aspect of perfecting the world that they would be able to participate in were they to eat this fruit.)
Now, the Torah does not speak disparagingly of anyone unless there is a reason to do so. It points out Moses’ error in order to teach us how severe an offense it is to speak disparagingly of others and how it is possible to make amends for doing so.
G-d showed Moses that it is possible to rectify the sin of slander by grasping the snake’s tail. The tail, the hindmost part of the animal, indicates lowliness and humility. By humbling our ego, we eliminate the haughtiness that makes us see faults in others.
--Daily Wisdom Volume 3