פירוש על שמות 7:17

ונהפכו לדם AND THEY SHALL BE TURNED INTO BLOOD — Because rain does not fall in Egypt but the Nile rises and irrigates the land and the Egyptians on this account worshipped the Nile, therefore God first smote their deity and afterwards smote them (Exodus Rabbah 9:9).

'בזאת תדע כי אני ה, to teach you who have said: “I do not know any god by the name of Hashem.” (5,2)

'בזאת תדע כי אני ה; I will turn something that never changes in nature and never ceases to flow, into something else. This will demonstrate My power over natural law.