כֹּ֚ה אָמַ֣ר יְהוָ֔ה בְּזֹ֣את תֵּדַ֔ע כִּ֖י אֲנִ֣י יְהוָ֑ה הִנֵּ֨ה אָנֹכִ֜י מַכֶּ֣ה ׀ בַּמַּטֶּ֣ה אֲשֶׁר־בְּיָדִ֗י עַל־הַמַּ֛יִם אֲשֶׁ֥ר בַּיְאֹ֖ר וְנֶהֶפְכ֥וּ לְדָֽם׃
Ganito ang sabi ng Walang-Hanggan: Sa pamamagitan nito ay malalaman mo na Ako ang Walang-Hanggan: narito, hahampasin ko ng tungkod na nasa aking kamay ang tubig na nasa Ilog, at ito ay magiging dugo.
Rashi
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).
Chizkuni
בזאת תדע, “by the way the punishment fits the crime you will know;” seeing that you have said: “I don’t know a deity called Hashem, I will give you proof of the existence and power of such a deity.” The precise meaning of the word תדע here is: “you will begin to know.”
Rashbam
'בזאת תדע כי אני ה, to teach you who have said: “I do not know any god by the name of Hashem.” (5,2)