Ramban, Exodus 15:25

And the reason “Adonai instructed him concerning a piece of wood” is that he showed him a piece of wood and said “cast this piece of wood into the water and it will be sweetened.” Since I cannot find a single use of the word “instructed” except regarding learning, [as in] (Proverbs 4:4) “And he instructed me, saying, teach me…” and likewise with all the other [instances], it seems to me that the simple meaning of the passage is that this wood would by its nature sweeten the water, as part of its inherent qualities, and this He taught to Moshe. And our sages have taught (Midrash Mechilta and Tanhuma, Exodus 15) that this tree was bitter, and it was doubly miraculous, as in [the miracle where] Elisha cast salt into the water (Kings 2 2:21). According to this [Midrash] the word “he instructed him” appears because this tree was not to be found in that place, and God guided [Moshe] to its location. Or He miraculously materialized it for him. Again I found in the “Yelamdenu” (Midrash Tanhuma Exodus 15), “And Adonai instructed him concerning a piece of wood”- It doesn’t say “He showed him” rather “He instructed him” [in Hebrew the difference is one letter] meaning, “He instructed him in His ways”. Meaning, He instructed and taught him the ways of the Holy One Blessed be He who sweetens the bitter with bitter. [Translation by Sareet Benayahu, edited for gender neutrality]