Rashi

Because of My knowledge that you are obstinate. Heb. מִדַּעְתִּי. Because I knew you, that you are obstinate, and if the miracle would come and I had not told of it before, you would say, “My idol performed them, and they were not from the Holy One, blessed be He.” Therefore, I told you from then; when it had not yet come to pass etc. מִדַּעְתִּי [although usually translated ‘from My knowledge,’ in this case,] ; the ‘mem’ denotes the giving of a reason, like ‘Because I knew.’ A similar form [is found in Deut. 7:8:], “For, because of the Lord’s love (מֵאַהֲבַת) for you, and because of His observing (וּמִשָּׁמְרוֹ) the oath.”